llvm-isel-fuzzer: Handle a subset of backend flags in the executable name

Here we add a secondary option parser to llvm-isel-fuzzer (and provide
it for use with other fuzzers). With this, you can copy the fuzzer to
a name like llvm-isel-fuzzer:aarch64-gisel for a fuzzer that fuzzer
AArch64 with GlobalISel enabled, or fuzzer:x86_64 to fuzz x86, with no
flags required. This should be useful for running these in OSS-Fuzz.

Note that this handrolls a subset of cl::opts to recognize, rather
than embedding a complete command parser for argv[0]. If we find we
really need the flexibility of handling arbitrary options at some
point we can rethink this.

llvm-svn: 315545