cmake: compiler: gcc: riscv: remove spurious Zmmul

Zmmul is a subset of M extension.
No need to enable Zmmul redundantly if M is already present.

Reference:
https://github.com/riscv/riscv-isa-manual/blob/449cd0c79ae91cc6f547726b8a6270f975be676e/src/m.tex#L173
https://github.com/riscv/riscv-isa-manual/issues/869



Signed-off-by: Marcin Szymczyk <marcin.szymczyk@nordicsemi.no>