runtime/race: building for iOS, but linking in object file built for macOS [1.16 backport]

[gopherbot]

@cherrymui requested issue #46931 to be considered for backport to the next 1.16 minor release.

@gopherbot please backport this to Go 1.16. This fixes a test currently failing on 1.16 branch.

[gopherbot]

Change https://golang.org/cl/372218 mentions this issue: [release-branch.go1.16] runtime/race: use race build tag on syso_test.go

[dmitshur]

Approved as a test-only fix. Only 1.16 needs this backport, since the fix is already present in Go 1.17.

[gopherbot]

Closed by merging 831d491 to release-branch.go1.16.