vendor/libc: a bunch of additions

All these additions are to allow Dear ImGui to be compiled natively.

vendor/libc/time.odin

@@ -0,0 +1,10 @@
+package odin_libc
+
+import "core:time"
+
+clock_t :: i64
+
+@(require, linkage="strong", link_name="clock")
+clock :: proc "c" () -> clock_t {
+	return time.tick_now()._nsec
+}