RISC-V: error handling for subset parsing

RISC-V: error handling for subset parsing

The parsing of both the argument of -march= and the operand(s) of
".option arch, ..." can fail. In such a case for .option (and equally
for .attribute) prior state should continue to be used, while for -march=
defaults should be put in place. In particular, extensions conflicting
with previously enabled extensions should not suddenly become available.

While re-indenting riscv_set_rvc() invocations, leverage that C implies
Zca (and hence a check for the latter suffices). Mirror that change to
riscv_set_arch() for consistency.

Reviewed-by: Jiawei <jiawei@iscas.ac.cn>