Build failures in powerpc ptrace selftests

[Seth Forshee <seth.forshee@canonical.com>]

With gcc 7 from Ubuntu 17.10 I'm getting the follwing error building the
ptrace selftests for powerpc:

  ptrace-tm-vsx.c: In function ‘tm_vsx’:
  ptrace-tm-vsx.c:42:2: error: PIC register clobbered by ‘r2’ in ‘asm’
    asm __volatile__(
    ^~~
  make[1]: *** [ptrace-tm-vsx] Error 1
  ptrace-tm-spd-vsx.c: In function ‘tm_spd_vsx’:
  ptrace-tm-spd-vsx.c:55:2: error: PIC register clobbered by ‘r2’ in ‘asm’
    asm __volatile__(
    ^~~
  make[1]: *** [ptrace-tm-spd-vsx] Error 1
  ptrace-tm-spr.c: In function ‘tm_spr’:
  ptrace-tm-spr.c:46:2: error: PIC register clobbered by ‘r2’ in ‘asm’
    asm __volatile__(
    ^~~

Best I can tell gcc now considers any inline asm which clobbers r2 an
error, and I get it even with -fno-pic. I'm not familiar with powerpc
asm, but it's not obvious to me why r2 is in the clobber list for any of
these.

Thanks,
Seth