Fix multiple SGEN_LOG to correctly scale elapsed time in usec. (mono/mono#16485)

Fix multiple SGEN_LOG to correctly scale elapsed time in usec. (mono/mono#16485)

Several SGEN_LOG entries logged usec based on result returend from TV_ELAPSED.
On SGEN's implementation of TV_ELAPSED, the result is not scaled to usec
but returned as 100ns ticks. This was not handled correctly by SGEN_LOG,
but other measures, like profile counters and SGEN binary protocol handles
it correctly.

Fix will adjust the SGEN_LOG calls currently presenting usec to correctly
scale the result. Fix also adds one additional logging measuring total
amount of time spend scanning thread data.

Commit migrated from https://github.com/mono/mono/commit/70aba13173840818246b470e425a6d2585e7672d