macos: support for -bind_at_load

[fanquake]

I would like to drop mold into a project that cross-compiles macOS binaries, however it does not yet support -bind_at_load:

-bind_at_load Sets a bit in the mach header of the resulting binary which tells dyld to bind all symbols when the binary is loaded, rather than lazily.

which the project include in its hardening flags.