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`brew install x86_64-elf-binutils` fails, because latest gcc version is
8 and `/usr/local/opt/gcc@7/` directory does not exist.
By designating gcc version at dependens_on argument, brew install
`gcc@7` if it does not exist.

Homebrew uess gcc-8 for gcc and no longer gcc-7. To continue
to use gcc-7 in homebrew, we will need to install gcc@7. gcc@7
will then be linked to '/usr/local/opt/gcc@7'.