Arch multilib detection issues

[laudiacay]

On line 221 of the install-deps script, it tries to install gcc-fortran, regardless of whether gcc or gcc-multilib was detected as the correct gcc version. To work properly, it needs to install gcc-fortran if it detects gcc, and gcc-fortran-multilib if it detects gcc-multilib. Right now it doesn't work on Arch when gcc-multilib is installed.

[hughperkins]

Good info. Thanks! PR?