Generalize bugpoint's concept of a "safe" backend, and add options

to allow the "safe" backend to be run with a different path, and/or
with different command-line options.

This enables the following use cases:
 - bugpoint llc against an llc command from a different build
 - bugpoint llc against the same llc with different command-line options
 - and more...

Also, document the existing "custom" interpreter options.

llvm-svn: 60681