Abort with appropriate message if executable pages cannot be allocated

Abort with appropriate message if executable pages cannot be allocated

This may happen e.g. if PaX MPROTECT feature is enabled.

* os_dep.c [!OS2 && !PCR && !AMIGA && !USE_WINALLOC && !MACOS
&& !DOS4GW && !NONSTOP && !SN_TARGET_PSP2 && !RTEMS && !__CC_ARM
&& MMAP_SUPPORTED] (GC_unix_mmap_get_mem): Treat result==MAP_FAILED as
unlikely; if the first mmap call returns MAP_FAILED then ABORT with the
appropriate message if GC_pages_executable (and errno is EACCES).