guix: disable gcov in base-linux-gcc
In a x86_64-linux-gnu build, this drops:
x86_64-linux-gnu/bin/x86_64-linux-gnu-gcov
x86_64-linux-gnu/bin/x86_64-linux-gnu-gcov-dump
x86_64-linux-gnu/bin/x86_64-linux-gnu-gcov-tool
x86_64-linux-gnu/lib/gcc/x86_64-linux-gnu/12.4.0: libgcov.a
For mingw-w64-gcc, --disable-gcov is currently passed for this target in Guix, due to issues with mingw-w64, see
https://github.com/fanquake/guix/blob/8bed031e58c9cf895d243790e7c80808b0075de7/gnu/packages/gcc.scm#L99-L102. However we'll add it in any case, in case it's re-enabled in future, when the underlying issues are fixed.