RISC-V: Adjust -march flags for binutils 2.38

RISC-V: Adjust -march flags for binutils 2.38

As of version 2.38 binutils defaults to ISA specification version
2019-12-13. This version of the specification has has separated the
the csr read/write (csrr*/csrw*) instructions and the fence.i from
the I extension and put them into separate Zicsr and Zifencei
extensions.

This implies that we have to adjust the -march flag passed to the
compiler accordingly.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>