powerpc/pmap: Add pmap_sync_icache() for radix pmap

DTrace pid provider writes to user space to set breakpoints. Failing to
sync the icache can lead to SIGTRAP. Radix pmap is the only one missing
a pmap_sync_icache() method, so the pid provider would only potentially
crash a process on a POWER9 or later system.

Approved by: re (cperciva)

(cherry picked from commit 6f0b2a235a133a381634ba9a7f5f477c64db9873)
(cherry picked from commit ed2c3bda43fbd0bfad9e45c78a59fff8a62422da)