Fix compilation errors on LoongArch.

[KatyushaScarlet]

1. The first commit removes some redundant and unused defines.
   They can cause compilation errors on LoongArch.
   Removing them will troubleshoot the problem.

In file included from /home/katyusha/build/ppsspp/Common/File/FileDescriptor.cpp:8:
/home/katyusha/build/ppsspp/Common/Net/SocketCompat.h:122:18: error: expected identifier before numeric constant
  122 | #define POLL_ERR 0x008 /* Error condition. */
      |                  ^~~~~
/home/katyusha/build/ppsspp/Common/Net/SocketCompat.h:122:18: error: expected ‘}’ before numeric constant
In file included from /usr/include/signal.h:58,
                 from /home/katyusha/build/ppsspp/Common/CommonFuncs.h:45,
                 from /home/katyusha/build/ppsspp/Common/Log.h:21,
                 from /home/katyusha/build/ppsspp/Common/File/FileDescriptor.cpp:11:
/usr/include/bits/siginfo-consts.h:196:1: note: to match this ‘{’
  196 | {
      | ^
/home/katyusha/build/ppsspp/Common/Net/SocketCompat.h:122:18: error: expected unqualified-id before numeric constant
  122 | #define POLL_ERR 0x008 /* Error condition. */
      |                  ^~~~~
In file included from /usr/include/features.h:511,
                 from /usr/include/errno.h:25,
                 from /home/katyusha/build/ppsspp/Common/File/FileDescriptor.cpp:3:
/usr/include/signal.h:393:1: error: expected declaration before ‘}’ token
  393 | __END_DECLS
      | ^~~~~~~~~~~

2. The second commit fixes the "memcpy not declared" error on LoongArch.
   This is just a temporary fix to using memcpy here.
   In a real scenario, it should also include lsxintrin.h and lasxintrin.h to perform SIMD optimizations on LoongArch.
   More information can be found here: https://jia.je/unofficial-loongarch-intrinsics-guide.

All changes have been tested on my LoongArch, ARM and X86 machines.

[hrydgard]

You can just do #include <cstring> without bothering with the ifdef.

[KatyushaScarlet]

You can just do #include <cstring> without bothering with the ifdef.

I did tried , but I got fatal error: cstring: no such file or directory