[test] Guard AVR test with CODEGENERATOR=AVR

This test is only relevant for the AVR target, so it should be guarded
with the CODEGENERATOR=AVR condition to avoid running it when the target
is not enabled in llvm-targets-to-build.

Author: kateinoigakukun

test/embedded/avr/testStdlibFunctioning.swift

@@ -1,6 +1,7 @@
 // RUN: %swift-frontend -typecheck %s -target avr-none-none-elf \
 // RUN:   -wmo -enable-experimental-feature Embedded
 // REQUIRES: embedded_stdlib_cross_compiling
+// REQUIRES: CODEGENERATOR=AVR
 
 import Swift