This patch fixes a number of bugs in ppc32 plt stub matching code.
1) The 4-insn stubs for shared libs and PIEs weren't matched.
2) The executable stub miscalculated PLT entry address (by oring a
   sign-extended quantity rather than adding).
3) Comments were not accurate.

In addition, the insn arrays are made const.

	* ppc-linux-tdep.c (powerpc32_plt_stub): Make const.
	(powerpc32_plt_stub_so_1): Rename from powerpc32_plt_stub_so.
	Remove nop.  Make const.  Comment.
	(powerpc32_plt_stub_so_2): New.
	(POWERPC32_PLT_CHECK_LEN): Rename from POWERPC32_PLT_STUB_LEN.
	Correct count.  Update uses.
	(ppc_skip_trampoline_code): Match powerpc32_plt_stub_so_2 too.
	Move common code reading PLT entry word.  Correct
	powerpc32_plt_stub PLT address calculation.
	* ppc64-tdep.c (ppc64_standard_linkage1): Make const.
	(ppc64_standard_linkage2, ppc64_standard_linkage3): Likewise.
	(ppc64_standard_linkage4, ppc64_standard_linkage5): Likewise.
	(ppc64_standard_linkage6, ppc64_standard_linkage7): Likewise.
	(ppc64_standard_linkage8): Likewise.
	* rs6000-tdep.c (ppc_insns_match_pattern): Make pattern const.
	Correct insns description.
	* ppc-tdep.h (ppc_insns_match_pattern): Update prototype.

Reviewed-By: Yao Qi <qiyaoltc@gmail.com>