RISC-V: correct alignment directive handling for text sections

RISC-V: correct alignment directive handling for text sections

.insn or data emitted inside text sections can lead to positions not
being at insn granularity. In such situations using alignment
directives should reliably enforce the requested alignment.
Specifically requests to align back to insn granularity may not be
ignored (where, as a subcase thereof, the ordering of ".option norvc"
and e.g. ".p2align 2" should not matter; so far the alignment directive
needs to come first to have any effect). Similarly ahead of emitting
NOPs alignment first needs to be forced back to insn granularity.

The new testcases actually point out a corner case issue in the
disassembler as well, which is being corrected at the same time: We
don't want to print "0x" without any subsequent digits.