cmd/link: enable DWARF combining on macOS ARM64

It appears the machoCalcStart function is meant to align the
segment, but it doesn't. Replace it with an actual alignment
calculation. Also, use the alignment from the configuration,
instead of hardcode.

With this fix we could enable DWARF combining on macOS ARM64.

Change-Id: I19ec771b77d752b83a54c53b6ee65af78a31b8ae
Reviewed-on: https://go-review.googlesource.com/c/go/+/253558


Reviewed-by: Than McIntosh <thanm@google.com>