gdb/testsuite: rename get_maint_bp_addr and move it to gdb-utils.exp

The get_maint_bp_addr procedure will be shared by other test suite, so
move it to gdb-utils.exp.

Following Andrew's suggestion, I renamed get_maint_bp_addr to
gdb_get_bp_addr, since it would have handled normal breakpoints in
addition to the internal ones.  Note that there is still room for
improvement in this procedure, which I indicated in comments nearby.