[Clang][AVR] Fix definitions on AVR target

Fix definitions for MCUs xmega16a4u and xmega128a4u to match avr-libc

Differential Revision: https://reviews.llvm.org/D109653

Author: LittleFox94

clang/lib/Basic/Targets/AVR.cpp

@@ -223,7 +223,7 @@ static MCUInfo AVRMcus[] = {
     {"atmega256rfr2", "__AVR_ATmega256RFR2__"},
     {"atmega2564rfr2", "__AVR_ATmega2564RFR2__"},
     {"atxmega16a4", "__AVR_ATxmega16A4__"},
-    {"atxmega16a4u", "__AVR_ATxmega16a4U__"},
+    {"atxmega16a4u", "__AVR_ATxmega16A4U__"},
     {"atxmega16c4", "__AVR_ATxmega16C4__"},
     {"atxmega16d4", "__AVR_ATxmega16D4__"},
     {"atxmega32a4", "__AVR_ATxmega32A4__"},
@@ -265,7 +265,7 @@ static MCUInfo AVRMcus[] = {
     {"atxmega384d3", "__AVR_ATxmega384D3__"},
     {"atxmega128a1", "__AVR_ATxmega128A1__"},
     {"atxmega128a1u", "__AVR_ATxmega128A1U__"},
-    {"atxmega128a4u", "__AVR_ATxmega128a4U__"},
+    {"atxmega128a4u", "__AVR_ATxmega128A4U__"},
     {"attiny4", "__AVR_ATtiny4__"},
     {"attiny5", "__AVR_ATtiny5__"},
     {"attiny9", "__AVR_ATtiny9__"},