Support registering hard coded pre arch mutators

Some pre arch mutators are hard coded into mutator.go and so could not
share code for registering those mutators between tests and runtime.
This change adds a new HardCodedPreArchMutators(RegisterMutatorFunc)
method to RegistrationContext which allows hard coded mutators to be
registered alongside other build components during init() and testing.

The method is treated as a noop on the InitRegistrationContext and
behaves just like the PreArchMutators(RegisterMutatorFunc) method on
the TestingContext.

Bug: 146540677
Test: m nothing
Change-Id: I6f8b1e2d54d9dc4e86f951ced61d1ee7b0fe4b2e