x86/EFI: meet further spec requirements for runtime calls

x86/EFI: meet further spec requirements for runtime calls

So far we didn't guarantee 16-byte alignment of the stack: While (so
far) we don't tell the compiler to use smaller alignment, we also don't
guarantee 16-byte alignment when establishing stack pointers for new
vCPU-s. Runtime service functions using SSE instructions may end with

Note that -mpreferred-stack-boundary=3 is can be used only from gcc 4.8
onwards, and -mincoming-stack-boundary=3 only from 5.3 onwards. It is
for that reason that an alternative approach (using higher than
necessary alignment) is being used when building with such older
compilers.

Furthermore we should avoid #MF to be raised on the FLDCW we do.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>