Add support for Apple Silicon (i.e., Darwin (not iOS), ARM64) with cross-compile linker

[mrutkows]

Step up to go 1.16 and support the new darwin-arm64 piar (note: on pre-1.16 golang darwin-arm64 meant iOS; now there is a new ios GOOS value).

Feature Tracking for rename of GOOS: golang/go#38485
1.16 release notes: https://golang.org/doc/go1.16

Need to turn cross compile linker on:
https://www.reddit.com/r/golang/comments/ltaega/cross_compiling_to_darwinarm64_apple_silicon_m1/