run: fix disabling the `c` extension

[fkokosinski]

This PR fixes the way that the c extension is disabled, i.e. removed from the arch ISA string. Without this change, for GCC versions where Zicsr and Zifencei constitute separate extensions, the following ISA string:

  rv32imc_zicsr_zifencei_zba_zbb_zbc_zbs

Would be changed to (note incorrect _zisr vs correct _zicsr):

  rv32im_zisr_zifenei_zba_zbb_zb_zbs

Instead of the expected:

  rv32im_zicsr_zifencei_zba_zbb_zbc_zbs