CI: add riscv64 job

[Bodigrim]

No description provided.

[clyring]

From the CI log:

  [36 of 47] Compiling Lift             ( tests/Lift.hs, tests/Lift.o, tests/Lift.dyn_o )
  /usr/bin/ld.bfd: cbits/fpstring.o: relocation R_RISCV_HI20 against `fps_compare' can not be used when making a shared object; recompile with -fPIC
  /usr/bin/ld.bfd: cbits/is-valid-utf8.o: relocation R_RISCV_HI20 against `__stack_chk_guard@@GLIBC_2.27' can not be used when making a shared object; recompile with -fPIC
  /usr/bin/ld.bfd: cbits/itoa.o: relocation R_RISCV_HI20 against `a local symbol' can not be used when making a shared object; recompile with -fPIC
  /usr/bin/ld.bfd: cbits/shortbytestring.o: relocation R_RISCV_HI20 against `a local symbol' can not be used when making a shared object; recompile with -fPIC
  /usr/bin/ld.bfd: unresolvable R_RISCV_CALL_PLT relocation against symbol `qsort@@GLIBC_2.27'
  /usr/bin/ld.bfd: unresolvable R_RISCV_HI20 relocation against symbol `__stack_chk_guard@@GLIBC_2.27'
  collect2: fatal error: ld terminated with signal 11 [Segmentation fault], core dumped
  compilation terminated.
  
  <no location info>: error:
      `riscv64-linux-gnu-gcc' failed in phase `Linker'. (Exit code: 1)

[Bodigrim]

Any riscv64 experts around? Maybe @supersven?