kgdbts: only use new asm-generic/ptrace.h api when needed

The new instruction_pointer_set helper is defined for people who have
converted to asm-generic/ptrace.h, so don't use it generally unless
the arch needs it (in which case it has been converted).  This should
fix building of kgdb tests for arches not yet converted.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Jason Wessel <jason.wessel@windriver.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>