Remove unneeded executable stack

Assembly routines in libthread and libmp use an executable stack by
default. This is not needed and can be removed using --noexecstack.

Patch by Andy Spencer

  http://code.swtch.com/plan9port/issue/76/
--- a/src/mkhdr
+++ b/src/mkhdr
@@ -13,7 +13,7 @@
 AR=9ar
 CFLAGS=
 LDFLAGS=
-AFLAGS=
+AFLAGS=--noexecstack
 CLEANFILES=
 NUKEFILES=
 YACC=9 yacc