runtime: put tracebackctxt C functions in .c file

runtime: put tracebackctxt C functions in .c file

Since tracebackctxt.go uses //export functions, the C functions can't be
externally visible in the C comment. The code was using attributes to
work around that, but that failed on Windows.

Change-Id: If4449fd8209a8998b4f6855ea89e5db1471b2981
Reviewed-on: https://go-review.googlesource.com/22786
Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>