[scudo] Support importing custom configuration

To define custom allocation, you only need to put the configuration in
custom_scudo_config.h and define two required aliases, then you will be
switched to the customized config and the tests will also run with your
configuration.

In this CL, we also have a minor refactor the structure of
configuration. Now the essential fields are put under the associated
hierarchy and which will make the defining new configuration easier.

Reviewed By: cferris

Differential Revision: https://reviews.llvm.org/D150481