Drop libgcc and default to libc++ for NetBSD/ARM on 6.99.40+.
Don't bother with keeping the old support for x86_64 in 6.99.23+, just
use a single range. Update test cases for the always-on --eh-frame-hdr.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208170 91177308-0d34-0410-b5e6-96231b3b80d8