[libc++] Make sure we re-export some missing libc++abi symbols from libc++

Summary:
Ensure we re-export __cxa_throw_bad_array_new_length and
__cxa_uncaught_exceptions from libc++, since they are now
provided by libc++abi.

Doing this allows us to stop linking explicitly against libc++abi in
the libc++abi tests, since libc++ re-exports all the necessary symbols.
However, there is one caveat to that. We don't want libc++ to re-export
__cxa_uncaught_exception (the singular form), since it's only provided
for backwards compatibility. Hence, for the single test where we check
this backwards compatibility, we explicitly link against libc++abi.

PR27405
PR22654

Reviewers: EricWF

Subscribers: christof, jkorous, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D60424

llvm-svn: 358690