[linux] Add linux_syscall

Add linux_syscall for both i386 and x86_64.

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

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

@@ -0,0 +1,45 @@
+
+	.section ".data"
+	.globl linux_errno
+
+linux_errno:	.int 0
+
+	.section ".text"
+	.code32
+	.globl linux_syscall
+	.type  linux_syscall, @function
+
+linux_syscall:
+	/* Save registers */
+	pushl	%ebx
+	pushl	%esi
+	pushl	%edi
+	pushl	%ebp
+
+	movl	20(%esp), %eax  // C arg1 -> syscall number
+	movl	24(%esp), %ebx  // C arg2 -> syscall arg1
+	movl	28(%esp), %ecx  // C arg3 -> syscall arg2
+	movl	32(%esp), %edx  // C arg4 -> syscall arg3
+	movl	36(%esp), %esi  // C arg5 -> syscall arg4
+	movl	40(%esp), %edi  // C arg6 -> syscall arg5
+	movl	44(%esp), %ebp  // C arg7 -> syscall arg6
+
+	int	$0x80
+
+	/* Restore registers */
+	popl	%ebp
+	popl	%edi
+	popl	%esi
+	popl	%ebx
+
+	cmpl	$-4095, %eax
+	jae	1f
+	ret
+
+1:
+	negl	%eax
+	movl	%eax, linux_errno
+	movl	$-1, %eax
+	ret
+
+	.size linux_syscall, . - linux_syscall