FastMemcpy{,_Avx}.c: Mark `memcpy()` as `dllimport` on MinGW and ming…

[lhmouse]

…w-w64 targets.

On MinGW and mingw-w64 targets, memcpy() is imported from MSVCRT.DLL. With regard to benchmarking purposes, we have to eliminate the overhead of implicit importation by specifying dllexport explicitly.

Reference: https://gcc.gnu.org/onlinedocs/gcc/Microsoft-Windows-Function-Attributes.html#index-dllimport-function-attribute Signed-off-by: Liu Hao [email protected]