Add a kdb show command to print arbitrary SPRs on PowerPC

Summary:
There is often a need at the debugger to print arbitrary special
purpose registers (SPRs) on PowerPC. Using a rewritable asm stub, print any SPR
provided on the command line.

Note, as there is no checking in this, attempting to print a nonexistent SPR
may cause a Program exception (illegal instruction, or boundedly undefined).

Note also that this relies on the kernel text pages being writable. If in the
future this is made not the case, this will need to be reworked.

Test Plan:
Printing the Processor Version Register (PVR, SPR 287):

db> show spr 11f
SPR 287(11f): 80240012

Differential Revision: https://reviews.freebsd.org/D7403