[RISCV] Fix canonical ordering of s* vs z* extensions in RISCVISAInfo

As noted in https://reviews.llvm.org/D148315, the ordering logic for
OrderedExtensionMap currently puts s* before z* extensions, but per the
ISA manual the correct order should be z* and then s* (with the
exception of zxm*, which are ordered after s*).

This patch fixes the ordering and adds a TODO for zxm*. The changes are
visible in the test case added in
a35e67fc which also demonstrates an
issue with the ordering of single letter extensions (which isn't
addressed in this patch).

This ordering matches the one used by GCC/binutils as well.

Differential Revision: https://reviews.llvm.org/D148615