Require targets to explicitly import pigweed variable definitions. This
moves out some necessary logic from the BUILDCONFIG.gn file.
Change-Id: If9c9fe5a680cc9d897a0e8a55889873be618ed84
Leave the pw_cpu_exception backend unset for two reasons:
1. pw_cpu_exception is not set up to be in the interrupt vector table,
so it does nothing anyway.
2. The pw_cpu_exception_armv7m tests do not pass in QEMU. The tests use
a divide-by-zero to trigger an exception that can be recovered from,
but it does not appear to trap in QEMU.
Change-Id: I22613f0e32b2fb8cf00a51b0da60554c27ec0e1a
Introduces the lm3s6965evb target designed to run inside QEMU. Not all
tests have been verified on this target, but the target builds and
successfully runs some of the tests.
Change-Id: Ic0c309c691096aa18b03bd85457195d6a2b49a0b
