Revert "Revert "[AVR] Allow specifying the CPU on the command line""

This reverts commit 7ac30e0f839fdab6d723ce2ef6a5b7a4cf03d150.

llvm-svn: 294282
diff --git a/clang/test/CodeGen/avr/target-cpu-defines/atmega328p.c b/clang/test/CodeGen/avr/target-cpu-defines/atmega328p.c
new file mode 100644
index 0000000..14b5e0d
--- /dev/null
+++ b/clang/test/CodeGen/avr/target-cpu-defines/atmega328p.c
@@ -0,0 +1,7 @@
+// REQUIRES: avr-registered-target
+// RUN: %clang_cc1 -E -dM -triple avr-unknown-unknown -target-cpu atmega328p /dev/null | FileCheck -match-full-lines %s
+
+// CHECK: #define AVR 1
+// CHECK: #define __AVR 1
+// CHECK: #define __AVR_ATmega328P 1
+// CHECK: #define __AVR__ 1