x86-64, gcc: Use -mpreferred-stack-boundary=3 if supported
On x86-64, the standard ABI requires alignment to 16 bytes. However, this is not actually necessary in the kernel (we don't do SSE except in very controlled ways); and furthermore, the standard kernel entry on x86-64 actually leaves the stack on an odd 8-byte boundary, which means that gcc will generate extra instructions to keep the stack *mis*aligned! gcc 4.8 adds an -mpreferred-stack-boundary=3 option to override this and lets us save some stack space and a handful of instructions. Note that this causes us to pass -mno-sse twice; this is redundant, but necessary since the cc-option test will fail unless -mno-sse is passed on the same command line. [ hpa: rewrote the patch description ] Signed-off-by:H.J. Lu <hjl.tools@gmail.com> Link: http://lkml.kernel.org/r/CAMe9rOqPfy3JcZRLaUeCjBe9BVY-P6e0uaSbMi5hvS-6WwQueg@mail.gmail.com Signed-off-by: