Now it is possible to build a c-archive as PIC on darwin/arm (this is
now the default). Then the system linker can link the binary using
the archive as PIE.

Fixes #12896.

Change-Id: Iad84131572422190f5fa036e7d71910dc155f155
Reviewed-on: https://go-review.googlesource.com/22461Reviewed-by: David Crawshaw <crawshaw@golang.org>