runtime: enable/disable SIGPROF if needed when profiling
This ensures that SIGPROF is handled correctly when using
runtime/pprof in a c-archive or c-shared library.
Separate profiler handling into pre-process changes and per-thread
changes. Simplify the Windows code slightly accordingly.
Fixes #18220.
Change-Id: I5060f7084c91ef0bbe797848978bdc527c312777
Reviewed-on: https://go-review.googlesource.com/34018
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>