scripts/gcc-plugins: consistently use HOSTCC

The GCC plugins are built using HOSTCC, but the path to the GCC plugins
headers is obtained using CC.  This can lead to interesting failures if
the host compiler and cross compiler are different versions, and the
host compiler uses the cross headers.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210923152811.406516-1-ross.burton@arm.com


Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>