cmd/link: round up default start address to alignment

cmd/link: round up default start address to alignment

If the -R flag (the segment alignment) is specified but the -T
flag (start address) is not, currently the default start address
may be under-aligned, and some math in the linker may be broken.
Round up the start address to align it.

Fixes #62064.

Change-Id: I3b98c9d0cf7d3cd944b9436a36808899d2e52572
Reviewed-on: https://go-review.googlesource.com/c/go/+/527822
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>