sim: fix fpu missing initializer warnings

sim: fix fpu missing initializer warnings

The current fpu code with externals enabled results in the warnings:
common/sim-fpu.c:2437: warning: missing initializer
common/sim-fpu.c:2437: warning: (near initialization for 'sim_fpu_zero.sign')
common/sim-fpu.c:2440: warning: missing initializer
common/sim-fpu.c:2440: warning: (near initialization for 'sim_fpu_qnan.sign')

So tweak the old style initializers to avoid these.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>