x86/build: Remove cc-option from -mskip-rax-setup

This has been supported in GCC since 5.1 and clang since 14.0. Now that x86
requires LLVM 15 or newer since

  7861640a ("x86/build: Raise the minimum LLVM version to 15.0.0"),

this flag can be unconditionally added, saving a compiler invocation.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=fbe575b652f5bdcc459f447a0e6f0e059996d4ef
Link: https://github.com/llvm/llvm-project/commit/a9fba2be35db674971382e38b99a31403444d9bf
Link: https://lore.kernel.org/20250814-x86-min-ver-cleanups-v1-5-ff7f19457523@kernel.org