[cc/coverage] Define a macro during coverage builds

Bug: http://b/186576313

During a coverage build, pass the __ANDROID_CLANG_COVERAGE__ macro to
native compilations.  This allows tools like dalvikvm and dex2oat, which
would otherwise call _exit(), to write profiles on coverage builds by
calling exit() instead.

Test: `m CLANG_COVERAGE=true NATIVE_COVERAGE_PATHS="art" nothing`
          and check compiler flags in out/soong/build.ninja.
Change-Id: I18315b89170abdd650e4c6c55577688348c42225

cc/coverage.go

@@ -96,7 +96,8 @@ func (cov *coverage) flags(ctx ModuleContext, flags Flags, deps PathDeps) (Flags
 			// flags that the module may use.
 			flags.Local.CFlags = append(flags.Local.CFlags, "-Wno-frame-larger-than=", "-O0")
 		} else if clangCoverage {
-			flags.Local.CommonFlags = append(flags.Local.CommonFlags, profileInstrFlag, "-fcoverage-mapping", "-Wno-pass-failed")
+			flags.Local.CommonFlags = append(flags.Local.CommonFlags, profileInstrFlag,
+				"-fcoverage-mapping", "-Wno-pass-failed", "-D__ANDROID_CLANG_COVERAGE__")
 		}
 	}