common/crc32c: stop using gp/tp as scratch in RISC-V Zbc CRC32C

common/crc32c: stop using gp/tp as scratch in RISC-V Zbc CRC32C

crc32c_zbc's fold-by-four loop used gp (x3) and tp (x4) as scratch for
two folding temporaries. Save/restore around the loop is not enough:
they are ABI-reserved, so a signal delivered mid-loop runs its handler
with a corrupted tp, and the first TLS access there faults.

On riscv64 this reliably crashes the crimson/seastore unittests
unittest-transaction-manager and unittest-omap-manager, where seastar's
stall-detector timer fires often: the process dies with SIGSEGV in the
linker's TLS path with garbage in gp/tp.

Fixes: https://tracker.ceph.com/issues/77904
Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>