make: revert to v7-m newlib
make: revert to v7-m newlib

For some reason using v7e-m causes the linker to move around sections.
See https://github.com/tock/elf2tab/pull/35 for more details. Until this
is all sorted out, fall back to v7-m which appears to work ok.

tock/libtock-c,tock/libtock-c,tock/libtock-c