Use compiler dependent spec files for mingw-w64

Back then in the gitian days when cross-compiling for Windows we used two i686-w64-mingw32-g++ -dumpspecs > ~/build/msvcr100.spec commands to extract the relevant spec file for making sure we use msvcr100 instead of msvcrt: one in the utils descriptor when building the compiler and one in the firefox descriptor when building the browser.

When moving to rbm those two commands got squashed into one and the resulting msvcr100.spec gets used for both the compiler and the browser. However, a crucial difference between both projects is that the GCC version used is not matching. Comparing the resulting spec file of the GCC used for the compiler (4.9.2) and the one used for the browser (6.4.0) shows a bunch of differences.

The system feels already quite fragile, so we should not bet on not seeing subtle bugs due to the used spec file in the browser not matching the respective compiler version. Rather, we should go back to the old behavior where we use the spec file bound to the compiler version used.