[PowerPC] Reject tdescs with VSX and no FPU or Altivec

[PowerPC] Reject tdescs with VSX and no FPU or Altivec

Currently rs6000_gdbarch_init will accept a tdesc with the
"org.gnu.gdb.power.vsx" feature but without the
"org.gnu.gdb.power.altivec" or "org.gnu.gdb.power.fpu".

It isn't clear from the standard features documentation that these are
requirements.  However, these tdescs would cause trouble in the VSX
pseudo-register functions, so this patch will cause them to be
rejected.

gdb/ChangeLog:
2018-10-26  Pedro Franco de Carvalho  <pedromfc@linux.ibm.com>

	* rs6000-tdep.c (rs6000_gdbarch_init): Reject tdescs with vsx but
	without altivec or fpu.