[Driver] Default to libc++ on FreeBSD

All supported FreeBSD releases use libc++, so default to it if the
target's major version is not specified.

Reviewed by:	dim, emaste
Differential Revision:	https://reviews.llvm.org/D77776

GitOrigin-RevId: 2dec2aa3ad083dbde838809f0e8c7ae31110e111
2 files changed