clang++: Always use --eh-frame-hdr on FreeBSD, even for -static

FreeBSD uses LLVM's libunwind on FreeBSD/arm64 today (and we expect to
use it more widely in the future) and it requires the EH frame segment
in static binaries.

Reviewed by: dim
Obtained from: Clang commit r266123
MFC after: 3 days
Relnotes: yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D7250