Adapt gcc's `contrib/gcc_update` script for use here

[cooljeanius]

GCC has a script in its contrib folder called gcc_update that fixes timestamps sometimes being off after a checkout:
https://github.com/cooljeanius/gcc/blob/master/contrib/gcc_update
Unfortunately it makes a lot of assumptions about the gcc-specific nature of the source tree's layout, so it can't just be copied over, but rather would require some modifications in order to be useful here...

[cooljeanius]

I might try doing a bit of this in the CI configs