Reland [RISCV] Fix gaps in IgnoreUnknown=true for RISCVISAInfo::parseArchString

Prior to this patch, unrecognised z/s/sx/x prefixed extensions were not
ignored when IgnoreUnknown=true.

The first version of this patch, a7313f83, incorrectly used
`!isSupportedExtension(Ext)` rather than `!isSupportedExtension(Name)`.
i.e. checked the full substring rather than the split out name, causing
incorrect behaviour when a version is specified. This was fixed and a
new test case addded.

Differential Revision: https://reviews.llvm.org/D145882