Do not set MCPU on Apple Silicon

[fxcoudert]

This fixes build from source on Apple Silicon: #44517

This MCPU:=apple-m1 leads to -mcpu=apple-m1 being passed to CC, CXX, and FC. However, apple-m1 is a valid option name for clang, but not for gcc and gfortran, and this leads to a build failure in OpenBLAS.

In fact, passing -mcpu=apple-m1 to the compilers is actually unnecessary: all macOS ARM compilers know what the baseline for support on this target is, and already compile code accordingly (this is true of clang and gcc/gfortran). So the simplest fix here is to remove this special case entirely. It was introduced in #36624, I think with the intent of allowing better optimisation. poke @Keno

[ViralBShah]

@yuyichao may also be able to chime in, and he noted along similar lines in #36624 (review)

[giordano]

I can confirm that even without specifying -mpcu, clang automatically sets it automatically to apple-m1 (see -target-cpu):

% echo 'int main(){}' | clang -x c - -###
Apple clang version 13.1.6 (clang-1316.0.21.2)
Target: arm64-apple-darwin21.4.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin
 "/Library/Developer/CommandLineTools/usr/bin/clang" "-cc1" "-triple" "arm64-apple-macosx12.0.0" "-Wundef-prefix=TARGET_OS_" "-Wdeprecated-objc-isa-usage" "-Werror=deprecated-objc-isa-usage" "-Werror=implicit-function-declaration" "-emit-obj" "-mrelax-all" "--mrelax-relocations" "-disable-free" "-disable-llvm-verifier" "-discard-value-names" "-main-file-name" "-" "-mrelocation-model" "pic" "-pic-level" "2" "-mframe-pointer=non-leaf" "-fno-strict-return" "-fno-rounding-math" "-munwind-tables" "-target-sdk-version=12.3" "-fvisibility-inlines-hidden-static-local-var" "-target-cpu" "apple-m1" "-target-feature" "+v8.5a" "-target-feature" "+fp-armv8" "-target-feature" "+neon" "-target-feature" "+crc" "-target-feature" "+crypto" "-target-feature" "+dotprod" "-target-feature" "+fp16fml" "-target-feature" "+ras" "-target-feature" "+lse" "-target-feature" "+rdm" "-target-feature" "+rcpc" "-target-feature" "+zcm" "-target-feature" "+zcz" "-target-feature" "+fullfp16" "-target-feature" "+sm4" "-target-feature" "+sha3" "-target-feature" "+sha2" "-target-feature" "+aes" "-target-abi" "darwinpcs" "-fallow-half-arguments-and-returns" "-debugger-tuning=lldb" "-target-linker-version" "762" "-resource-dir" "/Library/Developer/CommandLineTools/usr/lib/clang/13.1.6" "-isysroot" "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk" "-I/usr/local/include" "-internal-isystem" "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/local/include" "-internal-isystem" "/Library/Developer/CommandLineTools/usr/lib/clang/13.1.6/include" "-internal-externc-isystem" "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include" "-internal-externc-isystem" "/Library/Developer/CommandLineTools/usr/include" "-Wno-reorder-init-list" "-Wno-implicit-int-float-conversion" "-Wno-c99-designator" "-Wno-final-dtor-non-final-class" "-Wno-extra-semi-stmt" "-Wno-misleading-indentation" "-Wno-quoted-include-in-framework-header" "-Wno-implicit-fallthrough" "-Wno-enum-enum-conversion" "-Wno-enum-float-conversion" "-Wno-elaborated-enum-base" "-Wno-reserved-identifier" "-Wno-gnu-folding-constant" "-Wno-objc-load-method" "-fdebug-compilation-dir=/Users/mose" "-ferror-limit" "19" "-stack-protector" "1" "-fstack-check" "-mdarwin-stkchk-strong-link" "-fblocks" "-fencode-extended-block-signature" "-fregister-global-dtors-with-atexit" "-fgnuc-version=4.2.1" "-fmax-type-align=16" "-fcommon" "-fcolor-diagnostics" "-clang-vendor-feature=+messageToSelfInClassMethodIdReturnType" "-clang-vendor-feature=+disableInferNewAvailabilityFromInit" "-clang-vendor-feature=+disableNonDependentMemberExprInCurrentInstantiation" "-fno-odr-hash-protocols" "-clang-vendor-feature=+enableAggressiveVLAFolding" "-clang-vendor-feature=+revert09abecef7bbf" "-clang-vendor-feature=+thisNoAlignAttr" "-clang-vendor-feature=+thisNoNullAttr" "-mllvm" "-disable-aligned-alloc-awareness=1" "-D__GCC_HAVE_DWARF2_CFI_ASM=1" "-o" "/var/folders/v2/hmy3kzgj4tb3xsy8qkltxd0r0000gn/T/--b864d5.o" "-x" "c" "-"
 "/Library/Developer/CommandLineTools/usr/bin/ld" "-demangle" "-lto_library" "/Library/Developer/CommandLineTools/usr/lib/libLTO.dylib" "-no_deduplicate" "-dynamic" "-arch" "arm64" "-platform_version" "macos" "12.0.0" "12.3" "-syslibroot" "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk" "-o" "a.out" "-L/usr/local/lib" "/var/folders/v2/hmy3kzgj4tb3xsy8qkltxd0r0000gn/T/--b864d5.o" "-lSystem" "/Library/Developer/CommandLineTools/usr/lib/clang/13.1.6/lib/darwin/libclang_rt.osx.a"

So the manual setting is probably redundant, and actually harmful for the source build.