Support coverage instrumentation for Linux host

Bug: http://b/77792074

- Add the libclang_rt.profile runtime libraries directly to the compile
command (for both host and target) instead of relying on the Clang
driver.
- Move the coverage mutator to PreDepsMutators so the mutation has
already happened when runtime libraries are added during dependence
computation.
- Factor out cc/config/toolchain to identify libclang_rt.profile modules
for the x86 and x86_64 host.

Test: make NATIVE_COVERAGE=true produces coverage-enabled host binaries.
Change-Id: I1ebc8cffdf11622bfc18199a57674672888b3a5f

cc/config/x86_linux_host.go

@@ -270,6 +270,14 @@ func (t *toolchainLinuxX8664) YasmFlags() string {
 	return "${config.LinuxX8664YasmFlags}"
 }
 
+func (t *toolchainLinuxX86) profileRuntimeLibrary() string {
+	return "libclang_rt.profile-i386"
+}
+
+func (t *toolchainLinuxX8664) profileRuntimeLibrary() string {
+	return "libclang_rt.profile-x86_64"
+}
+
 func (t *toolchainLinux) AvailableLibraries() []string {
 	return linuxAvailableLibraries
 }