cc-wrapper: don't export CPP

CPP defaults to $(CC) -E and setting it explicitly seems to cause
issues in a number of packages like gcc and gdb.

https://www.gnu.org/software/make/manual/html_node/Implicit-Variables.html

pkgs/build-support/cc-wrapper/setup-hook.sh

@@ -139,7 +139,6 @@ export ${role}CC=@named_cc@
 export ${role}CXX=@named_cxx@
 
 for CMD in \
-    cpp \
     ar as nm objcopy ranlib strip strings size ld windres
 do
     if