MFC r332649: lld: add a __FreeBSD_version-style identifier to version

This will faciliate a WITH_SYSTEM_LINKER option.

MFC r332803:
lld: use correct number of digits in __FreeBSD_version-style ID

__FreeBSD_version-style IDs should have 5 digits following the major.

Also, use 11xxxxx for stable/11.

Sponsored by: The FreeBSD Foundation