tests: use warning output from rustc to catch missing compression

Using `ld.lld` may have been clever, but that was getting the /system/
ld.lld, not one we may have built as part of building llvm. By using the
warning message coming directly from rustc we now correctly skip the
zlib and zstd tests when the support is missing.

tests/run-make/compressed-debuginfo/Makefile

@@ -9,9 +9,7 @@ include ../tools.mk
 all: zlib zstandard
 
 zlib:
- $(RUSTC) --crate-name=foo --crate-type=lib --emit=obj -C debuginfo=full -Z debuginfo-compression=zlib foo.rs
- test "`ld.lld --compress-debug-sections=zlib 2>&1 | sed 's/.*unknown.*zlib/missing/'`" = missing || readelf -t $(TMPDIR)/foo.o | grep -q ZLIB
+ test "`$(RUSTC) --crate-name=foo --crate-type=lib --emit=obj -C debuginfo=full -Z debuginfo-compression=zlib foo.rs 2>&1 | sed 's/.*unknown.*zlib.*/missing/' | head -n 1`" = missing || readelf -t $(TMPDIR)/foo.o | grep -q ZLIB
 
 zstandard:
- $(RUSTC) --crate-name=foo --crate-type=lib --emit=obj -C debuginfo=full -Z debuginfo-compression=zstd foo.rs
- test "`ld.lld --compress-debug-sections=zstd 2>&1 | sed 's/.*unknown.*zstd/missing/'`" = missing || readelf -t $(TMPDIR)/foo.o | grep -q ZST
+ test "`$(RUSTC) --crate-name=foo --crate-type=lib --emit=obj -C debuginfo=full -Z debuginfo-compression=zstd foo.rs 2>&1 | sed 's/.*unknown.*zstd.*/missing/' | head -n 1`" = missing || readelf -t $(TMPDIR)/foo.o | grep -q ZST