Refactor ExtUtils::Embed to work with miniperl.

Refactor ExtUtils::Embed to work with miniperl.

Remove the use of FileHandle, which relies on IO, and XS module.
Only load Getopt::Std if it is needed (in xsinit()), to avoid needing to add
Getopt::Std to lib/buildcustomize.pl
Require File::Spec instead of using it, as it exports nothing, so there is no
benefit to using it (but it costs a BEGIN block).