Native Coverage support in Soong (gcov)

This is configured the same as make -- a global NATIVE_COVERAGE=true
flag to allow native coverage, then COVERAGE_PATHS=path1,path2,... to
turn it on for certain paths.

There are .gcnodir files exported to Make and saved in $OUT/coverage/...
files which are `ar` archives containing all of the compiler-produced
.gcno files for a particular executable / shared library.

Unlike the Make implementation, this only passes links the helper
library (automatically through --coverage) when one of the object files
or static libraries being used actually has coverage enabled.

Host support is currently disabled, since we set -nodefaultlibs, which
prevents libclang_rt.profile-*.a from being picked up automatically.

Bug: 32749731
Test: NATIVE_COVERAGE=true COVERAGE_PATHS=system/core/libcutils m -j libbacktrace libutils tombstoned
      $OUT/coverage/system/lib*/libcutils.gcnodir looks correct (self)
      $OUT/coverage/system/lib*/libbacktrace.gcnodir looks correct (static)
      $OUT/coverage/system/lib*/libutils.gcnodir doesn't exist (shared)
      $OUT/coverage/system/bin/tombstoned.gcnodir looks correct (executable)
Test: NATIVE_COVERAGE=true COVERAGE_PATHS=external/libcxxabi m -j libc++
      Confirm that $OUT/coverage/system/lib*/libc++.gcnodir looks correct (whole_static_libs)
Change-Id: I48aaa0ba8d76e50e9c2d1151421c0c6dc8ed79a9

android/variable.go

@@ -131,6 +131,9 @@ type productVariables struct {
 	ClangTidy  *bool   `json:",omitempty"`
 	TidyChecks *string `json:",omitempty"`
 
+	NativeCoverage *bool     `json:",omitempty"`
+	CoveragePaths  *[]string `json:",omitempty"`
+
 	DevicePrefer32BitExecutables *bool `json:",omitempty"`
 	HostPrefer32BitExecutables   *bool `json:",omitempty"`