Use x86_64 instructions for targetting 64-bit CPUs

[rilysh]

Hello,
In xbegin and xend functions, this program uses some older i386 instructions. For a proper x64-based CPU, if there's an alternative implementation won't it be better to use them instead?
For example, if we define the architecture (e.g. 64-bit) then run 64-bit instruction otherwise fallback to i386 one.

Refers

Am-I-affected-by-Meltdown/assembly_utils.hh

Line 49 in b6f8076

__asm__ __volatile__(".byte 0xc7,0xf8 ; .long 0" : "+a" (ret) :: "memory");

Am-I-affected-by-Meltdown/assembly_utils.hh

Line 55 in b6f8076

__asm__ __volatile__(".byte 0x0f,0x01,0xd5" ::: "memory");