bug#54626: gcc-4.7 and gcc-4.8 compilation failure in x64

I've managed to compile gcc-4.7 using a really old commit (v0.16.0) with guix time-machine but nowadays is broken.

bug#54626: gcc-4.7 and gcc-4.8 compilation failure in x64

Hi, I'm digging on old compiler versions for bootstrapping purposes and I realized that gcc 4.7 and 4.8 packages don't compile on x64. We have been able to reproduce the failure of gcc 4.7 but we didn't test the 4.8 error that far. In gcc-4.7 the error is the following: ./md-unwind-support.h: