Suppress -arch switches in the output of ExtUtils::Embed.

Suppress -arch switches in the output of ExtUtils::Embed.

We previously found out that OS X's standard perl installation tries to put
-arch switches into Perl link commands, evidently in hopes of building
universal binaries.  But it doesn't work to add such switches in plperl's
link step if they weren't being used earlier, so this is basically
unworkable.  When using gcc the result is only some warnings; but LLVM
fails entirely, so this issue isn't as cosmetic as we originally thought.
Hence, back-patch commit d69a419e682c2d39c2355105a7e5e2b90357c8f0 into
pre-9.0 branches.