Avoid -Wnarrowing warnings in ppc64-tdep.c

Avoid -Wnarrowing warnings in ppc64-tdep.c

This avoids -Wnarrowing warnings in ppc64-tdep.c, by adding a few
casts to unsigned.

gdb/ChangeLog
2018-08-27  Tom Tromey  <tom@tromey.com>

	* ppc64-tdep.c (insn_d, insn_ds, insn_xfx): Add casts to
	unsigned.
	(ppc64_standard_linkage1, ppc64_standard_linkage2)
	(ppc64_standard_linkage3, ppc64_standard_linkage4)
	(ppc64_standard_linkage5, ppc64_standard_linkage6)
	(ppc64_standard_linkage7, ppc64_standard_linkage8): Add casts to
	unsigned.