runtime: do not cpu profile idle threads on windows
Currently this leads to a significant skew towards 'etext' entry,
since all idle threads are profiled every tick.
Before:
Total: 66608 samples
63188 94.9% 94.9% 63188 94.9% etext
278 0.4% 95.3% 278 0.4% sweepspan
216 0.3% 95.6% 448 0.7% runtime.mallocgc
122 0.2% 95.8% 122 0.2% scanblock
113 0.2% 96.0% 113 0.2% net/textproto.canonicalMIMEHeaderKey
After:
Total: 8008 samples
3949 49.3% 49.3% 3949 49.3% etext
231 2.9% 52.2% 231 2.9% scanblock
211 2.6% 54.8% 211 2.6% runtime.cas64
182 2.3% 57.1% 408 5.1% runtime.mallocgc
178 2.2% 59.3% 178 2.2% runtime.atomicload64
LGTM=alex.brainman
R=golang-codereviews, alex.brainman
CC=golang-codereviews
https://golang.org/cl/
61250043