[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/x86_64/core/linux/linux_syscall.S

@@ -0,0 +1,33 @@
+
+	.section ".data"
+	.globl linux_errno
+
+linux_errno:	.int 0
+
+	.section ".text"
+	.code64
+	.globl linux_syscall
+	.type  linux_syscall, @function
+
+linux_syscall:
+	movq	%rdi, %rax    // C arg1 -> syscall number
+	movq	%rsi, %rdi    // C arg2 -> syscall arg1
+	movq	%rdx, %rsi    // C arg3 -> syscall arg2
+	movq	%rcx, %rdx    // C arg4 -> syscall arg3
+	movq	%r8, %r10     // C arg5 -> syscall arg4
+	movq	%r9, %r8      // C arg6 -> syscall arg5
+	movq	8(%rsp), %r9  // C arg7 -> syscall arg6
+
+	syscall
+
+	cmpq	$-4095, %rax
+	jae	1f
+	ret
+
+1:
+	negq	%rax
+	movl	%eax, linux_errno
+	movq	$-1, %rax
+	ret
+
+	.size linux_syscall, . - linux_syscall