newlib: fix preprocessor checks

Restore the call to AC_NO_EXECUTABLES -- I naively assumed in commit
2e9aa5f56cc26a411014a7f788423c670cfb5646 ("newlib: update preprocessor
configure checks") that checking for a preprocessor would not involve
linking code.  Unfortunately, autoconf will implicitly check that the
compiler "works" before allowing it to be used, and that involves a
link test, and that fails because newlib provides the C library which
is needed to pass a link test.

There is some code in NEWLIB_CONFIGURE specifically to help mitigate
these, but it's not kicking in here for some reason, so let's just add
the AC_NO_EXECUTABLES call back until we can unwind that custom logic.

Additionally, we have to call AC_PROG_CPP explicitly.  This was being
invoked later on, but only in the use_libtool=yes codepath, and that
is almost never enabled.

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