[linux] Add linuxprefix

Add a minimal _start required to run main.

Signed-off-by: Piotr Jaroszyński <p.jaroszynski@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/prefix/linuxprefix.S

+#include <linux/unistd.h>
+
+	.section ".text"
+	.code32
+	.globl _start
+	.type _start, @function
+
+_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	main
+
+	movl	%eax, %ebx // rc -> syscall arg1
+	movl	$__NR_exit, %eax
+	int	$0x80
+
+	.size _start, . - _start

src/arch/x86_64/prefix/linuxprefix.S

+#include <linux/unistd.h>
+
+	.section ".text"
+	.code64
+	.globl _start
+	.type _start, @function
+
+_start:
+	xorq	%rbp, %rbp
+
+	popq	%rdi       // argc -> C arg1
+	movq	%rsp, %rsi // argv -> C arg2
+
+	andq	$~15, %rsp // 16-byte align the stack
+
+	call	main
+
+	movq	%rax, %rdi // rc -> syscall arg1
+	movq	$__NR_exit, %rax
+	syscall
+
+	.size _start, . - _start