go/build: recognize '.sx' as equivalent of '.S'
On case insensitive filesystems, '.S' is interpreted as '.s' so,
providing option to use '.sx' extension for '.S' files as an alternative.
Fixes #32434
Change-Id: Ie2f7e5e2f3f12690ce18659e30ca94252a8f7bfc
GitHub-Last-Rev:
dcca989ec41ddc1d06ea509b78dce7d70bc996ed
GitHub-Pull-Request: golang/go#32557
Reviewed-on: https://go-review.googlesource.com/c/go/+/181699
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>