[cmake] Unconditionally set the force flag when codesigning

The Darwin linker now defaults to ad hoc signing binaries when targeting
Apple Silicon. This creates a problem when configuring targets that must
be built with entitlements: we either need to add -Wl,-no_adhoc_codesign
when building the target, or sign with the force flag set to allow
replacing a pre-existing signature.

Unconditionally force-signing is the more convenient solution. This
doesn't require a ld64 version check, and it's a much less invasive
cmake change.

Patch by Fred Riss!

rdar://70237254

Differential Revision: https://reviews.llvm.org/D89343