[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/x86_64/prefix/linuxprefix.S

@@ -0,0 +1,22 @@
+#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