cmd/internal: disable OSQRT on GOARM=5

OSQRT currently produces incorrect results when used on arm with softfloat.
Disable it on GOARM=5 until the actual problem is found and fixed.

Updates #10641

Change-Id: Ia6f6879fbbb05cb24399c2feee93c1be21113e73
Reviewed-on: https://go-review.googlesource.com/9524


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>