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