debug/pe, cgo: add windows support

R=rsc, mattn
CC=golang-dev
https://golang.org/cl/1976045
diff --git a/src/Make.pkg b/src/Make.pkg
index 10454c7..62fb68d 100644
--- a/src/Make.pkg
+++ b/src/Make.pkg
@@ -141,6 +141,7 @@
 _CGO_LDFLAGS_freebsd=-shared -lpthread -lm
 _CGO_LDFLAGS_linux=-shared -lpthread -lm
 _CGO_LDFLAGS_darwin=-dynamiclib -Wl,-undefined,dynamic_lookup
+_CGO_LDFLAGS_windows=-shared -lm -mthreads
 
 # Compile x.cgo4.c with gcc to make package_x.so.