Core: Drop custom copymem/zeromem defines

We've been using standard C library functions `memcpy`/`memset` for these since
2016 with 67f65f6639.

There was still the possibility for third-party platform ports to override the
definitions with a custom header, but this doesn't seem useful anymore.

Backport of #48239.

platform/android/export/export.cpp

@@ -2662,7 +2662,7 @@ public:
 					continue;
 				r_command_line_flags.resize(base + 4 + length);
 				encode_uint32(length, &r_command_line_flags.write[base]);
-				copymem(&r_command_line_flags.write[base + 4], command_line_argument.ptr(), length);
+				memcpy(&r_command_line_flags.write[base + 4], command_line_argument.ptr(), length);
 			}
 		}
 	}