Right now it concatenates it with the existing options and then appends
it to that list, fix it to simply append it as is, same as it is done
with the other variables.
Tested by running the following command which includes gcc options:
$ make check RUNTESTFLAGS="--tool_opts '-Werror'"
Without this patch, all the tests fail. With it, the test succeed.
Inspecting the logs shows that -Werror was indeed used when compiling
the test sources.
This eliminates all the *_win32.c tests in favor of the tests they were
branched from, and expands test coverage to run many more tests on
stdcall, thiscall, and fastcall.
This same mechanism also supports testing any other target that has
multiple ABIs.
This commons up code from libffi.call/call.exp and
libffi.special/special.exp, unifies the optimization option matrix
between the two, and makes it easier to add more axes to the matrix
in the future.
