Don't use CPP in Configurations/unix-Makefile.tmpl

We started using $(CPP) instead of $(CC) -E, with the assumption that
CPP would be predefined.  This is, however, not always true, and
rather depends on the 'make' implementation.  Furthermore, on
platforms where CPP=cpp or something else other than '$(CC) -E',
there's a risk that it won't understand machine specific flags that we
pass to it.  So it turns out that trying to use $(CPP) was a mistake,
and we therefore revert that use back to using $(CC) -E directly.

Fixes #5867

Note: this affects config targets that use Alpha, ARM, IA64, MIPS,
s390x or SPARC assembler modules.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5872)