X86-64: Add -mskip-rax-setup

X86-64: Add -mskip-rax-setup

The Linux kernel never passes floating point arguments around, vararg
functions or not. Hence no vector registers are ever used when calling a
vararg function.  But gcc still dutifully emits an "xor %eax,%eax" before
each and every call of a vararg function.  Since no callee use that for
anything, these instructions are redundant.

This patch adds the -mskip-rax-setup option to skip setting up RAX
register when SSE is disabled and there are no variable arguments passed
in vector registers.  Since RAX register is used to avoid unnecessarily
saving vector registers on stack when passing variable arguments, the
impacts of this option are callees may waste some stack space, misbehave
or jump to a random location.  GCC 4.4 or newer don't those issues,
regardless the RAX register value since they don't check the RAX register
value when SSE is disabled.

gcc/

	* config/i386/i386.c (ix86_expand_call): Skip setting up RAX
	register for -mskip-rax-setup when there are no parameters
	passed in vector registers.
	* config/i386/i386.opt (mskip-rax-setup): New option.
	* doc/invoke.texi: Document -mskip-rax-setup.

gcc/testsuite/

	* gcc.target/i386/amd64-abi-7.c: New tests.
	* gcc.target/i386/amd64-abi-8.c: Likwise.
	* gcc.target/i386/amd64-abi-9.c: Likwise.