Disable P10 optimizations on IBMi

IBMi does not yet support prefixed instructions, p10 features need
to be disabled until OS support is available.

Change-Id: Idca7d6ebd791b06ef8f1f8419badd1a3db0f277f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3562980Reviewed-by: Junliang Yan <junyan@redhat.com>
Commit-Queue: Milad Farazmand <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/main@{#79718}

src/codegen/ppc/assembler-ppc.cc

@@ -76,7 +76,13 @@ void CpuFeatures::ProbeImpl(bool cross_compile) {
 #else
   base::CPU cpu;
   if (cpu.part() == base::CPU::kPPCPower10) {
+// IBMi does not yet support prefixed instructions introduced on Power10.
+// Run on P9 mode until OS adds support.
+#if defined(__PASE__)
+    supported_ |= (1u << PPC_9_PLUS);
+#else
     supported_ |= (1u << PPC_10_PLUS);
+#endif
   } else if (cpu.part() == base::CPU::kPPCPower9) {
     supported_ |= (1u << PPC_9_PLUS);
   } else if (cpu.part() == base::CPU::kPPCPower8) {