Update two options to my proposed syntax for user-facing driver options:

  -gcc-toolchain foo  ->  --gcc-toolchain=foo
  -target foo         ->  --target=foo

I've added legacy aliases for the original spellings. I've updated the
canonical tests to check both spellings, and switched all of the
-gcc-toolchain usages elsewhere in the test suite to use the new one.
I've updated some of the usages of -target to the new syntax, but will
finish that in a separate entirely mechanical change once I'm sure this
won't get rolled back for some reason (It touches a *huge* number of RUN
lines in the test suite unsurprisingly).

A nice result is that the three most common flags I end up using when
doing cross compiles are all now consistent: --target=, --sysroot=, and
--gcc-toolchain=.