cmd/link: link dynamic library automatically

cgo_import_dynamic pragma indicates a symbol is imported from a
dynamic library. Currently, the linker does not actually link
against the dynamic library, so we have to "force" it by using

//go:cgo_import_dynamic _ _ "dylib"

syntax, which links in the library unconditionally.

This CL changes it to link in the library automatically when a
symbol is imported from the library, without using the "force"
syntax. (The "force" syntax is still supported.)

Remove the unconditional imports in the runtime. Now,
Security.framework and CoreFoundation.framework are only linked
when the x509 package is imported (or otherwise specified).

Fixes #40727.

Change-Id: Ied36b1f621cdcc5dc4a8f497cdf1c554a182d0e0
Reviewed-on: https://go-review.googlesource.com/c/go/+/248333


Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>