macos: avoid mprotect checks for JIT pages (#105)

Starting with macOS 11.2, mprotect calls for RWX pages will fail
in Apple Silicon, even if the page was granted permission and it was
requested the MAP_JIT flag, to better reflect the fact that the page
returned by mmap wasn't really RWX.

In macOS, there is an implementation for the executable allocator since
e87e1cc (macos: add BigSur support to execalloc (#90), 2020-11-30) that
flips the bits as needed, so this extra safeward is no longer needed.

HardenedBSD seems to be the last implementation of PaX that still lies,
so restrict the code only to that platform.

Fixes: #99

sljit_src/sljitExecAllocator.c

@@ -192,10 +192,13 @@ static SLJIT_INLINE void* alloc_chunk(sljit_uw size)
 	if (retval == MAP_FAILED)
 		return NULL;
 
+#ifdef __FreeBSD__
+        /* HardenedBSD's mmap lies, so check permissions again */
 	if (mprotect(retval, size, PROT_READ | PROT_WRITE | PROT_EXEC) < 0) {
 		munmap(retval, size);
 		return NULL;
 	}
+#endif /* FreeBSD */
 
 	SLJIT_UPDATE_WX_FLAGS(retval, (uint8_t *)retval + size, 0);