Recognize and skip 64-bit PowerPC Linux linkage functions.

Recognize and skip 64-bit PowerPC Linux linkage functions.
* ppc-linux-tdep.c (insn_d, insn_ds, insn_xfx, read_insn, struct
insn_pattern, insns_match_pattern, d_field, ds_field): New
functions, macros, and types for working with PPC instructions.
(ppc64_standard_linkage, PPC64_STANDARD_LINKAGE_LEN,
ppc64_in_solib_call_trampoline, ppc64_standard_linkage_target,
ppc64_skip_trampoline_code): New functions, variables, and macros
for recognizing and skipping linkage functions.
(ppc_linux_init_abi): Use ppc64_in_solib_call_trampoline and
ppc64_skip_trampoline_code for the 64-bit PowerPC Linux ABI.