Revert r315545 "llvm-isel-fuzzer: Handle a subset of backend flags in the executable name"

It broke some tests on Windows:

Failing Tests (4):
    LLVM :: tools/llvm-isel-fuzzer/execname-options.ll
    LLVM :: tools/llvm-isel-fuzzer/missing-triple.ll
    LLVM :: tools/llvm-isel-fuzzer/x86-empty-bc.ll
    LLVM :: tools/llvm-isel-fuzzer/x86-empty.ll

> 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: 315554