arcv: update bspeek and bspop encoding to reflect the XARCV 1.8 spec#98

MichielDerhaeg wants to merge 2 commits intoarc-2026.03foss-for-synopsys-dwc-arc-processors/binutils-gdb:arc-2026.03from

michiel/xarcv_bitstream_updatefoss-for-synopsys-dwc-arc-processors/binutils-gdb:michiel/xarcv_bitstream_updateCopy head branch name to clipboard