x86/build: Remove -pipe from KBUILD_CFLAGS

Commit 77b0bf55 ("kbuild/Makefile: Prepare for using macros in
inline assembly code to work around asm() related GCC inlining bugs")
added -Wa,- to KBUILD_CFLAGS, which breaks compiling with Clang (hangs
indefinitely at compiling init/main.o). This happens because while Clang
accepts -pipe (and has it documented in its list of supported flags), it
silently ignores it after this 2010 commit (thanks to Nick Desaulniers
for tracking this down), meaning that gas just infinitely waits for
stdin and never receives it.

https://github.com/llvm-mirror/clang/commit/c19a12dc3d441bec62eed55e312b76c12d6d9022

Initially, I had suggested just add -Wa,- to KBUILD_CFLAGS when GCC was
being used but that was before realizing it is because Clang doesn't do
anything with -pipe. H. Peter Anvin suggested checking to see if -pipe
gives us any gains out of GCC. Turns out it might actually be hurting:

With -pipe:

real    3m40.813s
real    3m44.449s
real    3m39.648s

Without -pipe:

real    3m38.492s
real    3m38.335s
real    3m38.975s

The issue of -Wa,- being passed along to gas without -pipe being
supported should still probably be fixed on the LLVM side (open issue:
https://bugs.llvm.org/show_bug.cgi?id=39410

) but this is not as much of
a workaround anymore since it helps both GCC and Clang.

Suggested-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nadav Amit <namit@vmware.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Kees Cook <keescook@chromium.org>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Link: https://github.com/ClangBuiltLinux/linux/issues/213
Link: https://lkml.kernel.org/r/20181023231125.27976-1-natechancellor@gmail.com