Use the libc++abi visibility macros instead of pragmas or using
visibility attributes directly. Clean up redundant attributes on
definitions (where the declarations already have visibility attributes
applied, from either libc++ or libc++abi headers).
Introduce _LIBCXXABI_WEAK as a drive-by cleanup, which matches the
semantics of _LIBCPP_WEAK.
No functional change. Tested by building on Linux before and after this
change and verifying that the list of exported symbols is identical.