Restore fastcontext support for big-endian PPC64

This commit restores support in fastcontext for the "function
descriptor" of the 64-bit PowerPC ELF ABI, which is common to both AIX
and Linux on big-endian PPC64.  It appears to have been
unintentionally removed along with AIX support in pull request #277
due to the (unfortunate) choice of `QTHREAD_PPC_ABI_AIX` to identify
this ABI.

Partially reverts 4f3031c

Signed-off-by: Paul H. Hargrove <[email protected]>

Author: PHHargrove

src/fastcontext/context.c

@@ -27,7 +27,11 @@ void INTERNAL qt_makectxt(uctxt_t *ucp, void (*func)(void), int argc, ...) {
   tos = (unsigned long *)ucp->uc_stack.ss_sp +
         ucp->uc_stack.ss_size / sizeof(unsigned long);
   sp = tos - 16;
+#if defined __BIG_ENDIAN__ || defined _BIG_ENDIAN
+  ucp->mc.pc = *(long*)func;
+#else
   ucp->mc.pc = (long)func;
+#endif
   ucp->mc.sp = (long)sp;
   va_start(arg, argc);
   ucp->mc.r3 = va_arg(arg, long);