[RISCV] Add X27 to SavedRegs when X26 is in SavedRegs for cm.push/pop (#92067)

cm.push can't save X26 without also saving X27. This removes two other
checks for this case.

This causes CFI to be emitted since X27 is now explicitly a callee saved
register.

The affected tests use inline assembly to clobber X26 rather than the
whole range of s0-s10.