DTrace: re-apply r249426 now that the underlying issues have been solved.

Merge change from illumos:

3519 DTrace fails to resolve const types from fbt
3520 dtrace internal error -- token type 316 is not a valid D
compilation token
3521 clean up dtrace unit tests

Illumos Revision: e98f46c

Reference:
https://www.illumos.org/issues/3519
https://www.illumos.org/issues/3520
https://www.illumos.org/issues/3521

Tested by: Fabian Keil
Obtained from: Illumos
MFC after: 1 month

DTrace: Revert r249426

This change actually depends on r249367 which had to be reverted

Pointy Hat: pfg