kdump/truss: decode new _umtx_op flags

In both cases, print the flag bits first followed by the command.

Output now looks something like this:

(ktrace)
_umtx_op(0x8605f7008,0xf<UMTX_OP_WAIT_UINT_PRIVATE>,0,0,0)
_umtx_op(0x9fffdce8,0x80000003<UMTX_OP__32BIT|UMTX_OP_WAKE>,0x1,0,0)

(truss)
_umtx_op(0x7fffffffda50,UMTX_OP_WAKE,0x1,0x0,0x0) = 0 (0x0)
_umtx_op(0x9fffdd08,UMTX_OP__32BIT|UMTX_OP_WAKE,0x1,0x0,0x0) = 0 (0x0)

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D27325