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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>