x86/Rules: Use -mskip-rax-setup if the compiler supports it

x86/Rules: Use -mskip-rax-setup if the compiler supports it

This option is available from GCC 5 onwards, and was specifically introduced
as an optimisation for Linux.  When using variadic functions, the caller needs
to know how many floating point arguments were passed.  Xen, like Linux,
doesn't uses floating point arguments, so doesn't need to emit code to inform
variadic functions such as printk() that there are zero arguments.

The net delta for a release build is:

  add/remove: 0/0 grow/shrink: 35/625 up/down: 603/-5489 (-4886)

with the single biggest change being:

  x86_emulate                               101933  101751    -182

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>