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>