Update FPCR test.

The test checks that writes to RES0 fields are ignored. This is fragile,
because those fields are sometimes allocated as the architecture is
updated.

This patch updates the RES0 field specification, but also runs that part
of the test only on simulator builds.

Change-Id: Ia667e32519f9e2639f0e46f34426c9a51d21a0e0