[AVR] Define __ELF__

This symbol is defined in avr-gcc. Because AVR normally uses the ELF
format, define the symbol unconditionally.

This patch is needed to get Clang to compile compiler-rt.

Differential Revision: https://reviews.llvm.org/D78117

Author: aykevl

clang/lib/Basic/Targets/AVR.cpp

@@ -300,6 +300,7 @@ void AVRTargetInfo::getTargetDefines(const LangOptions &Opts,
   Builder.defineMacro("AVR");
   Builder.defineMacro("__AVR");
   Builder.defineMacro("__AVR__");
+  Builder.defineMacro("__ELF__");
 
   if (!this->CPU.empty()) {
     auto It = llvm::find_if(

clang/test/CodeGen/avr/target-cpu-defines/atmega328p.c

@@ -5,3 +5,4 @@
 // CHECK: #define __AVR 1
 // CHECK: #define __AVR_ATmega328P__ 1
 // CHECK: #define __AVR__ 1
+// CHECK: #define __ELF__ 1

clang/test/CodeGen/avr/target-cpu-defines/attiny104.c

@@ -5,3 +5,4 @@
 // CHECK: #define __AVR 1
 // CHECK: #define __AVR_ATtiny104__ 1
 // CHECK: #define __AVR__ 1
+// CHECK: #define __ELF__ 1

clang/test/CodeGen/avr/target-cpu-defines/common.c

@@ -4,3 +4,4 @@
 // CHECK: #define AVR 1
 // CHECK: #define __AVR 1
 // CHECK: #define __AVR__ 1
+// CHECK: #define __ELF__ 1