Use MSVC inline assembly only on (32-bit) X86

Currently this used the _MSC_VER && !WIN64 to enable the inline
assembly, which still tried to use this code on windows on arm.
Using _MSC_VER && _M_IX86 is enough since _M_IX86 is defined only
when targeting 32 bit x86, not for x64.
Merge pull request #105 from mstorsjo/msvc-disable-inline-asm-on-arm

Use MSVC inline assembly only on (32-bit) X86