Update core:sync/sync2 to have a generic Futex interface, and implement the calls appropriately for each platform

core/sync/sync2/primitives_linux.odin

@@ -2,14 +2,9 @@
 //+private
 package sync2
 
-// TODO(bill): remove libc
-foreign import libc "system:c"
+import "core:intrinsics"
 
 _current_thread_id :: proc "contextless" () -> int {
-	foreign libc {
-		syscall :: proc(number: i32, #c_vararg args: ..any) -> i32 ---
-	}
-
 	SYS_GETTID :: 186
-	return int(syscall(SYS_GETTID))
+	return int(intrinsics.syscall(SYS_GETTID))
 }