powerpc64: Print current MSR on printtrap()

Summary:
Print current MSR on printtrap(). Currently, printtrap just prints srr1, which
contains part of the MSR prior to the exception. I find useful to dump the
current value of the MSR, since it changes when there is an interruption.

With this patch, this is the new printtrap model:

handled user trap:

exception       = 0x700 (program)
srr0            = 0x100008a0 (0x100008a0)
srr1            = 0x800000000002f032
current msr     = 0x8000000000009032
lr              = 0x1000089c (0x1000089c)
curthread       = 0x7a50000

pid = 714, comm = ttrap2

Submitted by: Breno Leitao
Reviewed by: nwhitehorn
Differential Revision: https://reviews.freebsd.org/D14600