Using julia_cmd during precompilation propagates JULIA_CPU_TARGET

[vchuravy]

For pkgimages I added the ability to set -C from julia_cmd flag.

julia/base/util.jl

Line 173 in a6ad9ea

cpu_target = unsafe_string(opts.cpu_target)

In the same motion I added the propagation of the -C flag to julia_cmd. Our current recommendation for people who want
to use multi-versioning for pkgimages is to set JULIA_CPU_TARGET="generic;sandybridge,-xsaveopt,clone_all;haswell,-rdrnd,base(1)",

I just came across a package (AMDGPU) in the wild that uses julia_cmd during precompilation...

cmd = Base.julia_cmd() = `/home/rocm-user/julia-1.9.0/bin/julia '-Cgeneric;sandybridge,-xsaveopt,clone_all;haswell,-rdrnd,base(1)' -J/home/rocm-user/julia-1.9.0/lib/julia/sys.so -g1 --color=yes --startup-file=no`
More than one command line CPU targets specified without a `--output-` flag specified

Not sure what the correct solution here is...