cmd/link: skip fallocate test if not supported, and adjust allocation size on darwin

cmd/link: skip fallocate test if not supported, and adjust allocation size on darwin

On Linux, the linker uses fallocate to preallocate the output
file storage. The underlying file system may not support
fallocate, causing the test to fail. Skip the test in this case.

On darwin, apparently F_PREALLOCATE allocates from the end of the
allocation instead of the logical end of the file. Adjust the
size calculation.

Fixes #39905.

Change-Id: I01e676737fd2619ebbdba05c7cf7f424ec27de35
Reviewed-on: https://go-review.googlesource.com/c/go/+/240618
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>