runtime: increase precision of GODEBUG=gctrace=1 times

[aclements]

Currently GODEBUG=gctrace=1 truncates cpu and clock times to ms. As a result, many of these times are almost always printed as 0, which is not useful for statistics. We should change this to print some decimal places if the whole part is small, down to at least microsecond precision. That will keep the numbers in user-friendly milliseconds, while still providing more precision for when it matters.

@RLH

[gopherbot]

CL https://golang.org/cl/10554 mentions this issue.