Fix ARM build with GCC trunk.

sysdeps/unix/sysv/linux/arm/unwind-resume.c and
sysdeps/unix/sysv/linux/arm/unwind-forcedunwind.c have static
variables that are written in C code but only read from toplevel asms.
Current GCC trunk now optimizes away such apparently write-only static
variables, so causing a build failure.  This patch marks those
variables with __attribute_used__ to avoid that optimization.

Tested that this fixes the build for ARM.

	* sysdeps/unix/sysv/linux/arm/unwind-forcedunwind.c
	(libgcc_s_resume): Use __attribute_used__.
	* sysdeps/unix/sysv/linux/arm/unwind-resume.c (libgcc_s_resume):
	Likewise.