Switch x86 syscall stubs over to the ENTER/END style of the ARM stubs.

Also update the x86 asm.h to support this; we need it for libm assembler
anyway.

Also clean up the _FBSDID hack in <sys/cdefs.h>.

Change-Id: Iababd977b8110ec022bf7c93f4d62ece47630e7c

libc/arch-x86/syscalls/gettid.S

@@ -1,12 +1,8 @@
 /* autogenerated by gensyscalls.py */
+#include <machine/asm.h>
 #include <sys/linux-syscalls.h>
 
-    .text
-    .type gettid, @function
-    .globl gettid
-    .align 4
-
-gettid:
+ENTRY(gettid)
     movl    $__NR_gettid, %eax
     int     $0x80
     cmpl    $-129, %eax
@@ -18,3 +14,4 @@ gettid:
     orl     $-1, %eax
 1:
     ret
+END(gettid)