Based on cl/69170045 by Elias Naur.

There are currently several schemes for acquiring a TLS
slot to save the g register. None of them appear to work
for android. The closest are linux and darwin.

Linux uses a linker TLS relocation. This is not supported
by the android linker.

Darwin uses a fixed offset, and calls pthread_key_create
until it gets the slot it wants. As the runtime loads
late in the android process lifecycle, after an
arbitrary number of other libraries, we cannot rely on
any particular slot being available.

So we call pthread_key_create, take the first slot we are
given, and put it in runtime.tlsg, which we turn into a
regular variable in cmd/ld.

Makes android/arm cgo binaries work.

LGTM=minux
R=elias.naur, minux, dave, josharian
CC=golang-codereviews
https://golang.org/cl/106380043