liblzma: Prevent warning for MSYS2 Windows build.

liblzma: Prevent warning for MSYS2 Windows build.

In lzma_memcmplen(), the <intrin.h> header file is only included if
_MSC_VER and _M_X64 are both defined but _BitScanForward64() was
previously used if _M_X64 was defined. GCC for MSYS2 defines _M_X64 but
not _MSC_VER so _BitScanForward64() was used without including
<intrin.h>.

Now, lzma_memcmplen() will use __builtin_ctzll() for MSYS2 GCC builds as
expected.