runtime: fast clock_gettime on FreeBSD, split getHPETTimecounter

runtime: fast clock_gettime on FreeBSD, split getHPETTimecounter

Call only initHPETTimecounter on the system stack.
Use O_CLOEXEC flag when opening the HPET device.

FreeBSD 12.3-RELEASE-p2, AMD FX-8300
paulzhol@relic:~/go/src/time % ~/gocode/bin/benchcmp old_hpet.txt new_hpet.txt
benchcmp is deprecated in favor of benchstat: https://pkg.go.dev/golang.org/x/perf/cmd/benchstat
benchmark                   old ns/op     new ns/op     delta
BenchmarkNow-8              1420          1397          -1.62%
BenchmarkNowUnixNano-8      1421          1404          -1.20%
BenchmarkNowUnixMilli-8     1423          1405          -1.26%
BenchmarkNowUnixMicro-8     1423          1404          -1.34%

Update #50947

Change-Id: I553b5427fb0b86d7e070af4516b36326bc0aaf00
Reviewed-on: https://go-review.googlesource.com/c/go/+/391856
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>