strub: disable on rl78

rl78 allocation of virtual registers to physical registers doesn't
operate on asm statements, and strub uses asm statements in the
runtime and in the generated code, to the point that the runtime
won't build.  Force strub disabled on that target.


for  gcc/ChangeLog

	* config/rl78/rl78.cc (TARGET_HAVE_STRUB_SUPPORT_FOR): Disable.