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
diff --git a/libc/include/sys/cdefs.h b/libc/include/sys/cdefs.h
index aa65b54..3617aba 100644
--- a/libc/include/sys/cdefs.h
+++ b/libc/include/sys/cdefs.h
@@ -377,11 +377,11 @@
 #define	__link_set_entry(set, idx)	(__link_set_begin(set)[idx])
 
 /*
- * Some of the recend FreeBSD sources used in Bionic need this.
+ * Some of the FreeBSD sources used in Bionic need this.
  * Originally, this is used to embed the rcs versions of each source file
  * in the generated binary. We certainly don't want this in Bionic.
  */
-#define	__FBSDID(s)	struct __hack
+#define __FBSDID(s) /* nothing */
 
 /*-
  * The following definitions are an extension of the behavior originally