Added prepare-libgmp.sh to repo and fixed multilib-generator string

[tovine]

Added fixes as mentioned in https://github.com/riscv/riscv-code-size-reduction/issues/183

[abukharmeh]

Thank you Torbjørn, Can we also change https://github.com/riscv/riscv-code-size-reduction/blob/8eb265260125f627c6e67ad4e8389cc43adff722/.gitmodules#L6 to url = https://github.com/openhwgroup/corev-gcc and https://github.com/riscv/riscv-code-size-reduction/blob/8eb265260125f627c6e67ad4e8389cc43adff722/.gitmodules#L9 to url = https://github.com/openhwgroup/corev-binutils-gdb as all bug fixes are being merged into these repos

[tovine]

Will do that, is there any particular branch of those repos I should track? In that case I can also update the submodules to track the latest versions there and make sure it still builds fine.

[abukharmeh]

Great, at the moment, the updates are in the default branch which is called development in both repos