Add support for Zalasr unratified extension

[mehnadnerd]

Based on #492.
I was able to test it with my hacked-up clang/spike and makefiles, so I don't think this can be merged until my patches for them are merged.
Specification is available at https://github.com/riscv/riscv-zalasr

[aswaterman]

Thanks. Agreed we need to wait to merge it.