Newlib headers for Canadian Crosses

These two patches have newlib match other libcs in the do_libc_start_files function by copying their headers from source to CT_HEADERS_DIR.

The second patch then makes copy_headers=y the default in gcc.sh for all modes, picking up the new headers copied from newlib by the first patch (contrary to the comment that there are no headers to copy for baremetal in the 'static' and 'baremetal' modes of do_cc_core_backend) 

Believe this should resolve #18