[build] Fix building with latest binutils

Signed-off-by: Bastian Blank <waldi@debian.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/prefix/linuxprefix.S

 	movl	$__NR_exit, %eax
 	int	$0x80
 
-	.size _start, . - _start
+	.size _linux_start, . - _linux_start

src/arch/i386/prefix/nbiprefix.S

 	int $0x19
 
 	.previous
-	.size	start, . - start
+	.size	_nbi_start, . - _nbi_start
 
 nbi_header_end:
 	.org 512