[RISC-V][MC] Fix mapping symbol section tracking on popSection() (#225131)
Previously, RISCVELFStreamer::changeSection() saved LastEMS and LastEmittedArch
under getPreviousSection().first instead of getCurrentSection().first. When
MCStreamer::popSection() switches back to a previous section,
getPreviousSection() already points to the destination section being restored
rather than the section being exited. This clobbered the destination section's
saved mapping symbol state and caused duplicate `$x<arch>` mapping symbols to
be emitted whenever returning to `.text`.
Use getCurrentSection().first instead, matching AArch64ELFStreamer and
ARMELFStreamer.
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Pull-Request: https://github.com/llvm/llvm-project/pull/225131