[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/x86_64/core/linux/linuxprefix.S

@@ -0,0 +1,25 @@
+#include <linux/unistd.h>
+
+	.section ".text"
+	.code64
+	.globl _linux_start
+	.type _linux_start, @function
+
+_linux_start:
+	xorq	%rbp, %rbp
+
+	popq	%rdi       // argc -> C arg1
+	movq	%rsp, %rsi // argv -> C arg2
+
+	andq	$~15, %rsp // 16-byte align the stack
+
+	call	save_args
+
+	/* Our main doesn't use any arguments */
+	call	main
+
+	movq	%rax, %rdi // rc -> syscall arg1
+	movq	$__NR_exit, %rax
+	syscall
+
+	.size _start, . - _start