arm: Fix warning in libgcc/config/arm/pr-support.c

I have noticed some warnings when building GCC for arm-eabi:
pr-support.c:110:7: warning: variable ‘set_pac_sp’ set but not used [-Wunused-but-set-variable]
pr-support.c:109:7: warning: variable ‘set_pac’ set but not used [-Wunused-but-set-variable]

This small patch avoids them by defining these two variables undef
TARGET_HAVE_PACBTI, like the code which actually uses them.

	libgcc/
	* config/arm/pr-support.c (__gnu_unwind_execute): Use
	TARGET_HAVE_PACBTI to define set_pac and set_pac_sp.