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/fgetxattr.S

@@ -1,12 +1,8 @@
 /* autogenerated by gensyscalls.py */
+#include <machine/asm.h>
 #include <sys/linux-syscalls.h>
 
-    .text
-    .type fgetxattr, @function
-    .globl fgetxattr
-    .align 4
-
-fgetxattr:
+ENTRY(fgetxattr)
     pushl   %ebx
     pushl   %ecx
     pushl   %edx
@@ -30,3 +26,4 @@ fgetxattr:
     popl    %ecx
     popl    %ebx
     ret
+END(fgetxattr)