Conditionalize the dependence of libunwind to x86-only to allow building
these ports on !x86.

libunwind is only available for x86; lang/ruby* already expresses this
correctly. Some of the rubygems did not: for the ones that already had
USE_RUBY, the dependency was overspecified in the first place.

Tested for no-harm on amd64.

While here, pet portlint where appropriate.

Approved by: portmgr (tier-2 blanket)