[libj6] Create libj6

Pull syscall code out of libc and create new libj6. This should
eventually become a vDSO, but for now it can still be a static lib.
Also renames all the _syscall_* symbol names to j6_*

src/libraries/j6/include/j6/syscalls.h

@@ -0,0 +1,15 @@
+#pragma once
+
+#include <j6/types.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#define SYSCALL(n, name, ...) j6_status_t j6_ ## name (__VA_ARGS__);
+#include "j6/tables/syscalls.inc"
+#undef SYSCALL
+
+#ifdef __cplusplus
+}
+#endif

src/libraries/j6/syscalls.s

@@ -0,0 +1,28 @@
+%macro SYSCALL 2
+	global j6_%1
+	j6_%1:
+		push rbp
+		mov rbp, rsp
+
+		; args should already be in rdi, etc, but rcx will
+		; get stomped, so stash it in r10, which isn't a
+		; callee-saved register, but also isn't used in the
+		; function call ABI.
+		mov r10, rcx
+
+		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
+%define SYSCALL(n, name, a, b, c, d) SYSCALL name, n
+%define SYSCALL(n, name, a, b, c, d, e) SYSCALL name, n
+
+%include "j6/tables/syscalls.inc"