Define _LIBCPP_CONSTINIT when building libc++abi

The macro is defined in libc++, but at a more recent version than the one we
currently use. Define it ourselves for now.

Bug: 1298070
Change-Id: I84368fd309c916997637a7b49ded2144a264e162
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3468387
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#972438}
NOKEYCHECK=True
GitOrigin-RevId: 3f516cfc18766fcb3d2308d2a043f6c40f5e0b4b
1 file changed