Support `sdk_version: "current"` for the NDK.

We need to fall back to the old CRT objects since we aren't generating
those yet.

Test: Created a test module using "current", checked that it linked
      the libs from current.
Bug: None

Change-Id: I5fe170d7b26154da8877672ac2acb7da0262fe38
diff --git a/cc/compiler.go b/cc/compiler.go
index c1040dd..1798f4e 100644
--- a/cc/compiler.go
+++ b/cc/compiler.go
@@ -183,8 +183,12 @@
 		// Traditionally this has come from android/api-level.h, but with the
 		// libc headers unified it must be set by the build system since we
 		// don't have per-API level copies of that header now.
+		version := ctx.sdkVersion()
+		if version == "current" {
+			version = "__ANDROID_API_FUTURE__"
+		}
 		flags.GlobalFlags = append(flags.GlobalFlags,
-			"-D__ANDROID_API__="+ctx.sdkVersion())
+			"-D__ANDROID_API__="+version)
 
 		// Until the full NDK has been migrated to using ndk_headers, we still
 		// need to add the legacy sysroot includes to get the full set of