cppcheck: Fix support for C language

Cppcheck automatically detects a source file's language based on its
extension and applies different rules to C and C++. Forcing language
to always C++ results in false positives when checking C files:

    templates/examples/mbedtls/main.c:239:34: warning: C-style pointer casting [cstyleCast]

Also, add C99 as standard for C files.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>