sigaction is called as part of library mode initializers
(_rt0_*_lib). Sigaction in turn calls getg, but on Android the TLS
offset for g has not been initialized and getg might return garbage.
Add a check for initialization before calling getg.
Fixes the golang.org/x/mobile/bind/java tests on amd64 and 386.
Fixes #31476
Change-Id: Id2c41fdc983239eca039b49a54b8853c5669d127
Reviewed-on: https://go-review.googlesource.com/c/go/+/172158 Reviewed-by: Ian Lance Taylor <iant@golang.org>