This commit leverages the runtime support for DWARF exception info added
in #27210 to enable unwinding by default on 64-bit MSVC. This also additionally
adds a few minor fixes here and there in the test harness and such to get
`make check` entirely passing on 64-bit MSVC:
* The invocation of `maketest.py` now works with spaces/quotes in CC
* debuginfo tests are disabled on MSVC
* A link error for librustc was hacked around (see #27438)
bbef8893f7