[linux] Avoid building Linux-specific prefix on non-Linux builds

Reported-by: David Yeske <dyeske@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/core/linux/linuxprefix.S

@@ -0,0 +1,28 @@
+#include <linux/unistd.h>
+
+	.section ".text"
+	.code32
+	.globl _linux_start
+	.type _linux_start, @function
+
+_linux_start:
+	xorl	%ebp, %ebp
+
+	popl	%esi       // save argc
+	movl	%esp, %edi // save argv
+
+	andl	$~15, %esp // 16-byte align the stack
+
+	pushl	%edi // argv -> C arg2
+	pushl	%esi // argc -> C arg1
+
+	call	save_args
+
+	/* Our main doesn't use any arguments */
+	call	main
+
+	movl	%eax, %ebx // rc -> syscall arg1
+	movl	$__NR_exit, %eax
+	int	$0x80
+
+	.size _linux_start, . - _linux_start