sys crate fails to cross compile for RISC-V

[fortanix]

sys crate fails to cross compile for RISC-V

[jethrogb]

      gcc: error: unrecognized argument in option ‘-mabi=ilp32’
      gcc: error: unrecognized argument in option ‘-mcmodel=medany’

These options aren't set by mbedtls-sys. So this is a generic issue with CMake/the cmake crate/MbedTLS.

[jethrogb]

Are you able to cross-compile any other CMake-based sources?

[piegamesde]

I tried compiling mbedtls and the error messages were definitely different (complaining about missing dependencies after a few file, and not about a "broken" compiler).

Which reinforces my supposition that the crate should work just fine if the build script knew which compiler to use.