[PATCH, newlib/ARM] Fix elf-nano.specs to work without -save-temps

[Thomas Preudhomme]

Hi,

it turns out that 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 [1]. This is because the driver only
use cpp_options spec for the external cpp tool, not for the integrated
one.

[1] Guess the options I used to test the change...

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.

Tested by running the following C program when building it with gcc or g++ with 
and without -specs=nano.specs and/or -save-temps:

#include <stdio.h>

int
main (void)
{
   printf ("%x\n", sizeof (struct _reent));
   return 0;
}


Is this ok for master?

Best regards,

Thomas
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