[mc] In compare_global_variables only compare values in the R/W segment

[mc] In compare_global_variables only compare values in the R/W segment

The algorithme is only relevant if the variable is in the R/W segment
otherwise the resulting pointer will be broken and might result in a
SIGSEV.

This happen if we try to compare a pointer on the .rodata (such as
__FUNCTION__).

Values are supposed to be constant and we do not expect to find
pointers to something which is not reachable by the global variables.