cmd/go, cmd/link, cmd/dist: re-enable plugin mode on darwin/amd64

1. remove broken verification
   The runtime check assumes that no-pcln symbol entry have zero value,
   but the linker emit no entries if the symbol is no-pcln.
   As a result, if there are no-pcln symbols at the very end of pcln
   table, it will panic.
2. correct condition of export
   Handle special chracters in pluginpath correcty.
   Export "go.itab.*", so different plugins can share the same itab.

Fixes #18190

Change-Id: Ia4f9c51d83ce8488a9470520f1ee9432802cfc1d
Reviewed-on: https://go-review.googlesource.com/61091


Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>