NX stacks on amd64 and powerpc

I added a support for non-executable stacks on amd64 and PowerPC
architectures some time ago, but did not enabled it. Passed time allowed
to fix some bugs in the implementation, and I consider it would be good
to have NX stacks enabled for architectures that support it.

I plan to commit the following knob twiddle in approximately a week.
Some ports, if any, that erronously handle PT_GNU_STACK phdr may break.

Anybody interested should and could test the change before the commit,
by setting sysctl kern.elf64.nxstack and kern.elf32.nxstack to 1.


diff --git a/sys/kern/imgact_elf.c b/sys/kern/imgact_elf.c
index b41741a..7358e40 100644
--- a/sys/kern/imgact_elf.c
+++ b/sys/kern/imgact_elf.c
_at__at_ -116,7 +115,12 _at__at_ static int elf_legacy_coredump = 0;
 SYSCTL_INT(_debug, OID_AUTO, __elfN(legacy_coredump), CTLFLAG_RW, 
     &elf_legacy_coredump, 0, "");
 
-static int __elfN(nxstack) = 0;
+static int __elfN(nxstack) =
+#if defined(__amd64__) || defined(__powerpc__) /* both 64 and 32 bit */
+	1;
+#else
+	0;
+#endif
 SYSCTL_INT(__CONCAT(_kern_elf, __ELF_WORD_SIZE), OID_AUTO,
     nxstack, CTLFLAG_RW, &__elfN(nxstack), 0,
     __XSTRING(__CONCAT(ELF, __ELF_WORD_SIZE)) ": enable non-executable stack");