PPC/s390: Delete ObjectTriple and support code

Port fc413158

Original Commit Message:

    ObjectTriple isn't used since f1ec44e2. Delete
    it, and simplify CEntryStub on all backends.

R=martyn.capewell@arm.com, joransiu@ca.ibm.com, jyan@ca.ibm.com, bjaideep@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=
LOG=N

Change-Id: I4897783bb848e016d93731585e6891033fa4d4cf
Reviewed-on: https://chromium-review.googlesource.com/714022Reviewed-by: Junliang Yan <jyan@ca.ibm.com>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#48476}

src/ppc/simulator-ppc.cc

@@ -1334,7 +1334,6 @@ void Simulator::SoftwareInterrupt(Instruction* instr) {
       int arg0_regnum = 3;
       intptr_t result_buffer = 0;
       bool uses_result_buffer =
-          redirection->type() == ExternalReference::BUILTIN_CALL_TRIPLE ||
           (redirection->type() == ExternalReference::BUILTIN_CALL_PAIR &&
            !ABI_RETURNS_OBJECT_PAIRS_IN_REGS);
       if (uses_result_buffer) {