Replace bc with shell arithmetic
Replace bc with shell arithmetic

The variable MEM_GIG was computed using bc, an arbitrary precision
calculator. That caused a numeric comparison that used that variable in
scripts/eosio_build_darwin.sh to fail when the scale was set to a value
greater than zero in the user's ~/.bc configuration file.

This commit fixes the problem by switching to shell arithmetic when
computing MEM_GIG.

EOSIO/eos,EOSIO/eos,EOSIO/eos,EOSIO/eos,EOSIO/eos