We're a bit inconsistent about which tool we use to preprocess source
files; in some places we use `$(CC) -E` whilst in others we use `cpp`.
This change forces all invocations of the C preprocessor to use the
first scheme, which ensures that the preprocessor behaves the same way
as the C compiler used when compiling C source files.
Change-Id: Iede2f25ff86ea8b43d7a523e32648058d5023832
Signed-off-by: Chris Kay <chris.kay@arm.com>
781cb31439