runtime/cgo: remove threadentry functions specialized for android

The specialized functions set up the g register using the pthread
API instead of setg_gcc, but the inittls functions have already
made sure setg_gcc works.

Updates #29674

Change-Id: Ie67c068d638af8b5823978ee839f6b61b2228996
Reviewed-on: https://go-review.googlesource.com/c/go/+/169797


Reviewed-by: Ian Lance Taylor <iant@golang.org>