[AVR][clang] Pass '-fno-use-init-array' to cc1 as default

On AVR, '.ctors' is used, not '.init_array'. Make this the default
unless specifically overridden by driver argument.

This matches gcc, and it matches the behavior in (e.g.) the NetBSD
driver (for certain OS variants).

Reviewed by: MaskRay

Differential Revision: https://reviews.llvm.org/D107610