sim/testsuite: Default global_cc_os and global_cc_works properly

There was an omission on 3e6dc39e "sim/testsuite: Set
global_cc_os also when no compiler is found"; global_cc_os
wasn't set for other than the primary target, which means
that the "unguarded" use of global_cc_os in
testsuite/cris/c/c.exp caused the dreaded "ERROR: can't read
"global_cc_os": no such variable" when e.g. configuring for
pru-elf and doing "make check-sim".  Better initializing
both variables at the top to default values, rather than
adding another single 'set global_cc_os ""', to reduce the
risk of not setting them properly if or when that
if-statement-chain is made longer.

sim/testsuite:
	* lib/sim-defs.exp (sim_init_toolchain): Default
	global_cc_os and global_cc_works properly, before if-chain.