The march and mtune flags are passed by the crossSystem gcc attributes,
which are placed *before* the flags to gcc. This may end up causing some
packages to break, but it is working fine so far.
In order to build the kernel without compressed instructions we need to
disable EFI support. We also need to leave the kernel to figure the
proper -march and -mabi flags, as otherwise it fails to build. Using the
option "CONFIG_RISCV_ISA_C n" disables the use of compressed
instructions.
