Remove _M_X86 in favour of _M_X86_64

[Zopolis4]

This removes an unnecessary definition, improves clarity and is functionally identical to before.

[Minty-Meeo]

In Hash.cpp, GetHash64_SSE42_CRC32 has 32-bit variant which get compiled instead of the 64-bit variant if _M_X86 is defined while _M_X86_64 is not. This is clearly legacy code from when Dolphin still supported 32-bit, though, so you could probably remove it.

[Pokechu22]

There seems to be library code that only uses _M_X86:

dolphin/Externals/zstd/lib/common/compiler.h

Lines 92 to 105 in b667931

	/* Enable runtime BMI2 dispatch based on the CPU.
	* Enabled for clang & gcc >=4.8 on x86 when BMI2 isn't enabled by default.
	*/
	#ifndef DYNAMIC_BMI2
	#if ((defined(__clang__) && __has_attribute(__target__)) \
	|| (defined(__GNUC__) \
	&& (__GNUC__ >= 5 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 8)))) \
	&& (defined(__x86_64__) || defined(_M_X86)) \
	&& !defined(__BMI2__)
	# define DYNAMIC_BMI2 1
	#else
	# define DYNAMIC_BMI2 0
	#endif
	#endif

dolphin/Externals/liblzma/config.h

Lines 154 to 157 in b667931

	#ifdef _M_X86
	/* Define to 1 if you have the <immintrin.h> header file. */
	#define HAVE_IMMINTRIN_H 1
	#endif

dolphin/Externals/liblzma/config.h

Lines 370 to 374 in b667931

	#ifdef _M_X86
	/* Define to 1 if the system supports fast unaligned access to 16-bit and
	32-bit integers. */
	#define TUKLIB_FAST_UNALIGNED_ACCESS 1
	#endif

I'm not sure what to do about that. I think you can just edit the liblzma config, and __x86_64__ may be automatically defined, though.

[Zopolis4]

I feel uneasy about it, but I've edited the liblzma files.

[Tilka]

Please rebase.