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

eclass/toolchain-funcs.eclass

@@ -59,7 +59,7 @@ tc-getCC() { tc-getPROG CC gcc "$@"; }
 # @FUNCTION: tc-getCPP
 # @USAGE: [toolchain prefix]
 # @RETURN: name of the C preprocessor
-tc-getCPP() { tc-getPROG CPP cpp "$@"; }
+tc-getCPP() { tc-getPROG CPP "${CC:-gcc} -E" "$@"; }
 # @FUNCTION: tc-getCXX
 # @USAGE: [toolchain prefix]
 # @RETURN: name of the C++ compiler
@@ -132,7 +132,7 @@ tc-getBUILD_CC() { tc-getBUILD_PROG CC gcc "$@"; }
 # @FUNCTION: tc-getBUILD_CPP
 # @USAGE: [toolchain prefix]
 # @RETURN: name of the C preprocessor for building binaries to run on the build machine
-tc-getBUILD_CPP() { tc-getBUILD_PROG CPP cpp "$@"; }
+tc-getBUILD_CPP() { tc-getBUILD_PROG CPP "$(tc-getBUILD_CC) -E" "$@"; }
 # @FUNCTION: tc-getBUILD_CXX
 # @USAGE: [toolchain prefix]
 # @RETURN: name of the C++ compiler for building binaries to run on the build machine