Use gdbarch methods for solib stuff on PowerPC Linux.

Use gdbarch methods for solib stuff on PowerPC Linux.
* config/powerpc/tm-linux.h (IN_SOLIB_CALL_TRAMPOLINE,
SKIP_TRAMPOLINE_CODE): #undef these, so the gdbarch methods will
show through.
* ppc-linux-tdep.c (ppc_linux_init_abi): Register
IN_SOLIB_CALL_TRAMPOLINE and SKIP_TRAMPOLINE_CODE methods here,
giving the same effect as the #definitions above.