[AVR] Fix __AVR_xxx macro definitions; authored by Peter Wu

Summary:
The -mmcu option for GCC sets macros like __AVR_ATmega328P__ (with the trailing
underscores), be sure to include these underscores for Clangs -mcpu option.

See "AVR Built-in Macros" in https://gcc.gnu.org/onlinedocs/gcc/AVR-Options.html

Reviewers: jroelofs, dylanmckay

Reviewed By: jroelofs, dylanmckay

Subscribers: efriedma, cfe-commits

Differential Revision: https://reviews.llvm.org/D29817

llvm-svn: 294869
diff --git a/clang/test/CodeGen/avr/target-cpu-defines/atmega328p.c b/clang/test/CodeGen/avr/target-cpu-defines/atmega328p.c
index 14b5e0d..83b0acc 100644
--- a/clang/test/CodeGen/avr/target-cpu-defines/atmega328p.c
+++ b/clang/test/CodeGen/avr/target-cpu-defines/atmega328p.c
@@ -3,5 +3,5 @@
 
 // CHECK: #define AVR 1
 // CHECK: #define __AVR 1
-// CHECK: #define __AVR_ATmega328P 1
+// CHECK: #define __AVR_ATmega328P__ 1
 // CHECK: #define __AVR__ 1