Coldfire ISA A doesn't include atomic instructions

[anjohnson]

Forwarding an issue that Joel discovered on RTEMS-uC5282, C++ code using std::atomic doesn't link because the GCC support is incomplete: Add support for std::atomic on Coldfire ISA A (uC5282 std::atomic_flag link error)

This probably means pvxs won't build for that target. Base and the older external modules only use the epicsAtomic APIs, which have a workaround.

I don't see pvxs' use of std::atomic as a problem, this gives us a solid reason to drop this Coldfire BSP in the EPICS 7.1 release series.