cmd/cgo: accept extra leading _ on __cgodebug_data for all object formats

The current Windows build breakage appears to be because
the Windows code should be looking for __cgodebug_data
not ___cgodebug_data. Dodge the question everywhere by
accepting both.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/19780043
1 file changed