Separate ELFTOOLCHAIN_BOOTSTRAP from BINUTILS_BOOTSTRAP

For most cases they are equivalent, but BINUTILS_BOOTSTRAP is a
BROKEN_OPTION on arm64 as the in-tree GNU binutils do not support it,
so we need a separate internal flag for ELF Tool Chain.

Reviewed by: andrew, brooks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3381