toolchain-funcs.eclass: Assume CPP="$(tc-getCC) -E" when unset, #582822

toolchain-funcs.eclass: Assume CPP="$(tc-getCC) -E" when unset, #582822

Modify the tc-getCPP and tc-getBUILD_CPP functions to use "$(tc-getCC)
-E" (i.e. the C compiler's preprocessing call) instead of falling back
to 'cpp'. This ensures that in environment with CC (and CXX) overriden
the correct compiler is used rather than the one selected by gcc-config,
which in turn fixes gcc version queries.

The alternative would be to always override CPP along with CC & CXX.
However, that is uncommon and is known to break some packages.

Bug: https://bugs.gentoo.org/show_bug.cgi?id=582822