[Driver][AVR] Fix warn_drv_avr_stdlib_not_linked condition

Many options (-fsyntax-only, -E, -S, etc) skip the link action phase which the
existing condition does not account for.

Since the code no longer specifies OPT_c, I think a single RUN line about -c
not leading to a warning is sufficient. Adding one for all of -E,
-fsyntax-only, -S would be excessive.

Reviewed By: benshi001

Differential Revision: https://reviews.llvm.org/D122553