Let MSVC force inline ZSTD_hashPtr() function

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ZSTD_hashPtr() function was not expanded by MSVC, led to low performance compared to GCC.

Issue #2314

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A test failed, seems to be due to this PR:

317compress/zstd_compress_internal.h:650:30: error: ‘ZSTD_hashPtr’ defined but not used [-Werror=unused-function]
318 FORCE_INLINE_TEMPLATE size_t ZSTD_hashPtr(const void* p, U32 hBits, U32 mls)
319                              ^
320cc1: all warnings being treated as errors
321In file included from compress/zstd_compress_superblock.c:18:0:
322compress/zstd_compress_internal.h:650:30: error: ‘ZSTD_hashPtr’ defined but not used [-Werror=unused-function]
323 FORCE_INLINE_TEMPLATE size_t ZSTD_hashPtr(const void* p, U32 hBits, U32 mls)
324                              ^
325cc1: all warnings being treated as errors
326In file included from compress/zstd_ldm.h:18:0,
327                 from compress/zstd_ldm.c:11:
328compress/zstd_compress_internal.h:650:30: error: ‘ZSTD_hashPtr’ defined but not used [-Werror=unused-function]
329 FORCE_INLINE_TEMPLATE size_t ZSTD_hashPtr(const void* p, U32 hBits, U32 mls)
330                              ^
331cc1: all warnings being treated as errors
332In file included from compress/zstdmt_compress.c:27:0:
333compress/zstd_compress_internal.h:650:30: error: ‘ZSTD_hashPtr’ defined but not used [-Werror=unused-function]
334 FORCE_INLINE_TEMPLATE size_t ZSTD_hashPtr(const void* p, U32 hBits, U32 mls)
335                              ^
336cc1: all warnings being treated as errors
337In file included from dictBuilder/fastcover.c:24:0:
338dictBuilder/../compress/zstd_compress_internal.h:650:30: error: ‘ZSTD_hashPtr’ defined but not used [-Werror=unused-function]
339 FORCE_INLINE_TEMPLATE size_t ZSTD_hashPtr(const void* p, U32 hBits, U32 mls)
340                              ^
341cc1: all warnings being treated as errors
342In file included from dictBuilder/zdict.c:51:0:
343dictBuilder/../compress/zstd_compress_internal.h:650:30: error: ‘ZSTD_hashPtr’ defined but not used [-Werror=unused-function]
344 FORCE_INLINE_TEMPLATE size_t ZSTD_hashPtr(const void* p, U32 hBits, U32 mls)
345                              ^
346cc1: all warnings being treated as errors
347Makefile:210: recipe for target 'libzstd.so.1.4.6' failed
348make[1]: *** [libzstd.so.1.4.6] Error 1
349make[1]: Leaving directory '/home/travis/build/facebook/zstd/lib'
350Makefile:63: recipe for target 'lib' failed
351make: *** [lib] Error 2

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Change FORCE_INLINE_TEMPLATE to MEM_STATIC FORCE_INLINE_ATTR

Some functions are using this definition:

MEM_STATIC FORCE_INLINE_ATTR
void ZSTD_wildcopy(void* dst, const void* src, ptrdiff_t length, ZSTD_overlap_e const ovtype)

MEM_STATIC FORCE_INLINE_ATTR size_t BIT_getUpperBits(size_t bitContainer, U32 const start)

MEM_STATIC FORCE_INLINE_ATTR size_t BIT_getMiddleBits(size_t bitContainer, U32 const start, U32 const nbBits)

This definition adds an __attribute__((unused)) for __GNUC__.
I tested, it speeded up for MSVC build.

MEM_STATIC:

#if defined(__GNUC__)
#  define MEM_STATIC static __inline __attribute__((unused))
#elif defined (__cplusplus) || (defined (__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L) /* C99 */)
#  define MEM_STATIC static inline
#elif defined(_MSC_VER)
#  define MEM_STATIC static __inline
#else
#  define MEM_STATIC static  /* this version may generate warnings for unused static functions; disable the relevant warning */
#endif