cmd/link: add -asan option

The -asan option causes the linker to link against the runtime/asan
package in order to use the C/C++ address sanitizer.

This CL passes tests but is not usable by itself.  The actual
runtime/asan package, and support for -asan in the go tool and the
compiler, and tests, are in separate CLs.

Updates #44853.

Change-Id: Ifc6046c1f75ba52777cbb3d937a4b66e91d5798d
Reviewed-on: https://go-review.googlesource.com/c/go/+/298610


Trust: fannie zhang <Fannie.Zhang@arm.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>