cmd/gomobile: fix CC, CXX compliers for ios builds
cmd/gomobile: fix CC, CXX compliers for ios builds

Fixes #golang/go#11339.

Change-Id: Ib6069d8471332a80251bea6e8740fc7a96a16404
Reviewed-on: https://go-review.googlesource.com/11614
Reviewed-by: David Crawshaw <a2f0bf29257797dd66800b4fbb9945c3e216c004@golang.org>

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