The new version breaks Win64 compilation with MinGW64

[agentzh]

The newly ported string hashing implementation breaks the build on 64-bit Windows using MinGW64:

C:\msys64\home\agentzh\work\openresty-1.19.3.1rc0\build\LuaJIT-2.1-20201002\src/x64/src/lj_str_hash_x64.h:158: undefined reference to `srandom'
C:\msys64\home\agentzh\work\openresty-1.19.3.1rc0\build\LuaJIT-2.1-20201002\src/x64/src/lj_str_hash_x64.h:167: undefined reference to `random'
C:\msys64\home\agentzh\work\openresty-1.19.3.1rc0\build\LuaJIT-2.1-20201002\src/x64/src/lj_str_hash_x64.h:168: undefined reference to `random'
C:\msys64\home\agentzh\work\openresty-1.19.3.1rc0\build\LuaJIT-2.1-20201002\src/x64/src/lj_str_hash_x64.h:179: undefined reference to `random'
C:\msys64\home\agentzh\work\openresty-1.19.3.1rc0\build\LuaJIT-2.1-20201002\src/x64/src/lj_str_hash_x64.h:179: undefined reference to `random'
collect2.exe: error: ld returned 1 exit status

@siddhesh Will you have a quick look at it? Thanks!