Define _LIBCPP_BUILDING_LIBRARY while building libc++abi

Ports https://github.com/llvm/llvm-project/commit/9c30bafd59bf which is
needed to keep std::unexpected_handler around (which libcxxabi uses
from libcxx) when building it with -std=c++20 (where
std::unexpected_handler got removed).

Bug: 1298070
Change-Id: Ia0c043cafc9fb36b23b9964af5e2162176ec139c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3469341
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Arthur Eubanks <aeubanks@google.com>
Commit-Queue: Arthur Eubanks <aeubanks@google.com>
Cr-Commit-Position: refs/heads/main@{#972026}
NOKEYCHECK=True
GitOrigin-RevId: d9c0da33531afd6923ad166575f85b09c9dfa198
1 file changed