selftests/powerpc: Fix build errors in powerpc ptrace selftests

GCC 7 will take "r2" in clobber list as an error and it will get
following build errors for powerpc ptrace selftests even with -fno-pic
option:
  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__(
    ^~~

Fix the build error by removing "r2" from the clobber list. None of
these asm blocks actually clobber r2.

Reported-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Simon Guo <wei.guo.simon@gmail.com>
Tested-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>