Fix executable memory allocation ifdefs and size
Created by: kbrenneman
The #ifdef's in u_execmem.c are no longer needed, since all of the OS-specific code got moved into utils_misc.c, and even that code should work on any POSIX system. The rest in u_execmem.c is just vestigial Win32 and fallback code.
Also, the allocation size is hard-coded to assume 64-byte dispatch stubs, which is too small for some architectures.