[MC][MachO] Change addrsig format + ensure its size is properly set

There were two problems with the previous setup:

1. We weren't setting its size, which caused problems when `__llvm_addrsig`
   wasn't the last section. In particular, `__debug_line` (if created) is
   generated and placed after `__llvm_addrsig`, and would result in an
   invalid object file w/ overlapping sections being emitted.

2. The symbol indices could be invalidated if e.g. `llvm-strip` ran on
   the object file. See discussion [here][1].

To fix both these issues, we use symbol relocations instead of encoding
symbol indices directly in the section contents. The section itself
doesn't contain any data. That sidesteps the layout problem in addition
to solving the second issue.

The corresponding LLD change to read in this new format: {D128938}.
It will fix the icf-safe.ll test failure on this diff.

[1]: https://discourse.llvm.org/t/problems-with-mach-o-address-significance-table-generation/63392/

Reviewed By: #lld-macho, alx32

Differential Revision: https://reviews.llvm.org/D127637