cmd/link/internal/ld: extract Mach-O load command parsing

We're going to need the ability to extract the LC_VERSION_MIN_* and
LC_BUILD_VERSION load commands. This CL adds peekMachoPlatform to do
that and in the process simplifies machoCombineDwarf.

While here, disable DWARF combining for Apple platforms other than
macOS (watchOS, tvOS, bridgeOS), not just iOS.

Updates #22395

Change-Id: I4862b0f15ccc87b7be1a6532b4d37b47c8f7f243
Reviewed-on: https://go-review.googlesource.com/c/go/+/168459


Reviewed-by: Ian Lance Taylor <iant@golang.org>