This is a followup to 2073b55e6b964cb8ca15e8c74cb32dac00f05f0d.
* gnu/build/cross-toolchain.scm (%gcc-include-paths): Switch back to
'C_INCLUDE_PATH' & co.
* gnu/packages/cross-base.scm (%gcc-include-paths): Likewise.
(cross-gcc-arguments): Remove 'treat-glibc-as-system-header' phase.
(cross-gcc)[native-inputs]: Reorder so that libc comes last.
[search-paths]: Add "include/c++" for 'CROSS_CPLUS_INCLUDE_PATH'.
* guix/build-system/gnu.scm (standard-cross-packages): Have "cross-gcc"
appear both for 'host and 'target.