use cmodel=medany when multilib build is enabled

[cfriedt]

Even when building the homebrew-riscv toolchain via the command below, there are a few different circumstances where certain relocations are unsupported.

brew install riscv-gnu-toolchain --enable-multilib

A number of tickets can be found here that highlight the various relocation errors: https://github.com/riscv/riscv-gnu-toolchain/issues?q=is%3Aissue+relocation+%22--with-cmodel=medany%22

I propose using --with-cmodel=medany additionally when --enable-multilib is passed in to brew, as that solved the relocation issues for me.

There is a 1-liner fixup forthcoming.

[cfriedt]

Examples of relocation errors of the sort described here can be found in riscv/riscv-gnu-toolchain#784 and riscv/riscv-gnu-toolchain#457

[sbeamer]

Hopefully fixed by #59