[Flang] Address review comment to add a LIT test. Also added comments to indicate that this macro is generic for PowerPC.

Author: DanielCChen

flang/lib/Frontend/CompilerInvocation.cpp

@@ -1339,6 +1339,8 @@ void CompilerInvocation::setDefaultPredefinitions() {
   case llvm::Triple::ArchType::ppcle:
   case llvm::Triple::ArchType::ppc64:
   case llvm::Triple::ArchType::ppc64le:
+    // '__powerpc__' is a generic macro for any PowerPC cases. e.g. Max integer
+    // size.
     fortranOptions.predefinitions.emplace_back("__powerpc__", "1");
     break;
   }

flang/test/Driver/predefined-macros-powerpc.f90

@@ -0,0 +1,11 @@
+! Test predefined macro for PowerPC architecture
+
+! RUN: %flang_fc1 -cpp -E %s | FileCheck %s
+! REQUIRES: target=powerpc{{.*}}
+
+! CHECK: integer :: var1 = 1
+
+#if __powerpc__
+  integer :: var1 = __powerpc__
+#endif
+end program