newlib: update preprocessor configure checks

newlib: update preprocessor configure checks

The nds32 & spu dirs are using compile tests to look for some
preprocessor defines, but we don't need to compile the code,
just preprocess it.  So switch to AC_PREPROC_IFELSE.

The sh dir is using a preprocessor test via grep, but let's
switch it to AC_PREPROC_IFELSE too to be consistent.

This should allow us to drop the uncommon AC_NO_EXECUTABLES call.