Fix elf-nano.specs to work without -save-temps

Fix elf-nano.specs to work without -save-temps

The changes in af272aca591fe1dc0f1be64ae5bda147ea98a047 only works when
using gcc/g++ with -E or -save-temps, otherwise newlib's newlib.h gets
used even if -specs=nano.specs is specified. This is because the driver
only use cpp_options spec for the external cpp tool, not for the
integrated one.

This patch uses instead cpp_unique_options which is used in all cases:
it is used directly when the integrated preprocessor is used, and
indirectly by expansion of cpp_options otherwise.