libstdc++: fix symbol version script for LLD

LLD is less tolerant of inconsistencies in the symbol version script.

• Add a ; on the last entry in a version block
• Remove duplicated symbols, retaining those in the earliest block

Reviewed by: bdrewery
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D10428