cc65: fix HEAD install; gcc.mak no longer exists.

The `-f make/gcc.mak` option is still required for the latest stable
release, but `make/gcc.mak` was made unnecessary and removed in cc65
commit 9fece990bea6525bec5afd8b60d1c731a4d13839 in 2013, so providing
that option for HEAD builds causes an error.

Closes #34639.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>