core/sys/unix: Add syscalls_linux.odin

Linux is in the unfortunate situation where the system call number is
architecture specific.  This consolidates the system call number
definitions in a single location, adds some wrappers, and hopefully
fixes the existing non-portable invocations of the syscall intrinsic.

core/sync/sync_linux.odin

@@ -1,11 +1,9 @@
 package sync
 
 import "core:sys/unix"
-import "core:intrinsics"
 
 current_thread_id :: proc "contextless" () -> int {
-	SYS_GETTID :: 186
-	return int(intrinsics.syscall(SYS_GETTID))
+	return unix.sys_gettid()
 }