arm64/disassem.c: Add detection of xzr and sp

Added support to distinguish between XZR/WZR and SP/WSP registers.

Add new OP_ flags to indicate if the instruction allows the use of SP
for a given register field. "wSP" and "SP" are removed from w_reg and
x_reg, and helper functions are introduced for this purpose of detecting
the correct name of the x31 register.

mhorne: While here, adjust some whitespace issues from a previous
commit.

Reviewed by: mhorne
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D39862