Extract LC_CODE_SIGNATURE related implementation out of LLD

Move the functionality in lld that handles writing of the LC_CODE_SIGNATURE load command and associated data section to a central reusable location.

This change is in preparation for another change that modifies llvm-objcopy to reproduce the LC_CODE_SIGNATURE load command and corresponding
data section to maintain the validity of signed macho object files passed through llvm-objcopy.

Reviewed By: #lld-macho, int3, oontvoo

Differential Revision: https://reviews.llvm.org/D109803