gnu: gcc-arm-none-eabi@7-2018-q2-update: Fix conflicting GCC headers.

This is a follow-up of 7f6d3f39 and merely
a copy-and-paste of Marius Bakke's fix.

* gnu/packages/embedded.scm (gcc-arm-none-eabi-7-2018-q2-update)[arguments]:
Add phase to remove the default compiler from CPLUS_INCLUDE_PATH.
[native-inputs]: Distinguish GCC-5.