In the normal case, sigFetchG just returns the G register. But in
the case that sigFetchG fetches the G from somewhere else, the G
register still holding an invalid value. Setg here to make sure
they match.

This is particularly useful because setGsignalStack, called by
adjustSignalStack from sigtrampgo before setg to gsignal,
accesses the G register.

Should fix #35249.

Change-Id: I64c85143cb05cdb2ecca7f9936dbd8bfec186c2d
Reviewed-on: https://go-review.googlesource.com/c/go/+/204441Reviewed-by: Ian Lance Taylor <iant@golang.org>