[RISCV] Use a lambda to avoid having the Support library depend on Option library.

RISCVISAInfo::toFeatures needs to allocate strings using
ArgList::MakeArgString, but toFeatures lives in Support and
MakeArgString lives in Option.

toFeature only has one caller, so the simple fix is to have that
caller pass a lamdba that wraps MakeArgString to break the
dependency.

Differential Revision: https://reviews.llvm.org/D112032