depends: enable lto support for Apple's ld64
Summary:
Note that this does not _enable_ lto by default in any way, only hooks up the machinery for -flto to work correctly. enable-lto-support is explicitly used for pinned-clang because we know it works. It is neither enabled nor disabled in the external clang case so that it can be auto-detected.
Backport of core PR19530.
Test Plan:
./contrib/teamcity/build-configurations.py build-osx
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D8784