clang: don't set -march for overridden target

If -target is explicitly passed to clang, we shouldn't pass our -march
value for the default target, because it probably won't exist for the
target being used.  Up until now, clang has been lenient with this,
but it's a hard error with clang 17, so since gcc.arch is always set
on aarch64, fixing this is a hard requirement for upgrading our
default clang to 17.

Before (with clang 17 on aarch64-linux):

	$ clang -target bpf -c -o /dev/null test.bpf.c
	clang: warning: ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Woption-ignored]
	clang: error: unsupported option '-march=' for target 'bpf'
	clang: warning: argument unused during compilation: '--gcc-toolchain=/nix/store/cngak08nb1yk44gnjipv5rg1ahh1xkax-gcc-13.2.0' [-Wunused-command-line-argument]

After:

	$ clang -target bpf -c -o /dev/null test.bpf.c
	clang: warning: ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Woption-ignored]
	clang: warning: argument unused during compilation: '--gcc-toolchain=/nix/store/cngak08nb1yk44gnjipv5rg1ahh1xkax-gcc-13.2.0' [-Wunused-command-line-argument]

0 files changed, 0 insertions, 0 deletions