libgo: use POSIX shell arithmetic expansion

Avoid bash-specific ((expression)) syntax.  As the bash syntax
converts a non-zero value to a zero status (and a zero value to a 1
status), and POSIX arithmetic expansion does not, we have to negate
the result.

Based on patch by Sören Tempel.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/419154