Fix some more issues

- ARM Neon headers are now exported in libc/isystem/

- stat() and access() now do a better job reporting which files are
  executable which ones aren't. They do this by reading the first two
  bytes in a file to see if it's `MZ` or `#!`.

libc/sysv/dos2errno.sh

@@ -47,7 +47,7 @@ dos	kNtErrorGenFailure              EACCES
 dos	kNtErrorGracefulDisconnect	EPIPE
 dos	kNtErrorHostDown                EHOSTUNREACH
 dos	kNtErrorHostUnreachable         EHOSTUNREACH
-dos	kNtErrorInsufficientBuffer	EFAULT
+dos	kNtErrorInsufficientBuffer	ENOBUFS
 dos	kNtErrorNoaccess                EFAULT
 dos	kNtErrorInvalidAddress          EADDRNOTAVAIL
 dos	kNtErrorNotAReparsePoint	EINVAL

libc/sysv/dos2errno/EFAULT.S

@@ -11,6 +11,5 @@
 	.globl	kDos2Errno.EFAULT
 	.type	kDos2Errno.EFAULT,@object
 kDos2Errno.EFAULT:
-	.e	kNtErrorInsufficientBuffer,EFAULT
 	.e	kNtErrorNoaccess,EFAULT
 	.e	WSAEFAULT,EFAULT

libc/sysv/dos2errno/ENOBUFS.S

@@ -0,0 +1,14 @@
+//	generated by libc/sysv/dos2errno.sh
+#include "libc/nt/errors.h"
+#ifndef __x86_64__
+        .end
+#endif
+	.macro	.e doscode systemv
+	.short	\doscode
+	.long	\systemv
+	.endm
+	.section .sort.rodata.dos2errno.2,"a",@progbits
+	.globl	kDos2Errno.ENOBUFS
+	.type	kDos2Errno.ENOBUFS,@object
+kDos2Errno.ENOBUFS:
+	.e	kNtErrorInsufficientBuffer,ENOBUFS