[libc] Created syscall trampolines in libc

Using syscalls.inc (moved to src/include) generate trampoline functions
for directly calling syscalls with libc functions.

src/libraries/libc/arch/x86_64/syscalls.s

@@ -0,0 +1,21 @@
+%macro SYSCALL 2
+	global _syscall_%1
+	_syscall_%1:
+		push rbp
+		mov rbp, rsp
+
+		; address of args should already be in rdi, etc
+		mov rax, %2
+		syscall
+		; result is now already in rax, so just return
+
+		pop rbp
+		ret
+%endmacro
+
+%define SYSCALL(n, name) SYSCALL name, n
+%define SYSCALL(n, name, a) SYSCALL name, n
+%define SYSCALL(n, name, a, b) SYSCALL name, n
+%define SYSCALL(n, name, a, b, c) SYSCALL name, n
+
+%include "syscalls.inc"

src/libraries/libc/include/j6libc/syscalls.h

@@ -0,0 +1,13 @@
+#pragma once
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#define SYSCALL(n, name, ...) j6_status_t _syscall_ ## name (__VA_ARGS__);
+#include "syscalls.inc"
+#undef SYSCALL
+
+#ifdef __cplusplus
+}
+#endif