x86/build: Move '-mindirect-branch-cs-prefix' out of GCC-only block

LLVM 16 will have support for this flag so move it out of the GCC-only
block to allow LLVM builds to take advantage of it.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://github.com/ClangBuiltLinux/linux/issues/1665
Link: https://github.com/llvm/llvm-project/commit/6f867f9102838ebe314c1f3661fdf95700386e5a
Link: https://lore.kernel.org/r/20230120165826.2469302-1-nathan@kernel.org