PPC: Fix '[strong] Function arity check should be based on required parameters'

lwa cannot encode unaligned displacements.  lwz is acceptable here
since the shift right will perform the sign-extension on PPC64 for us.

R=arv@chromium.org, dstence@us.ibm.com, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1134913004

Cr-Commit-Position: refs/heads/master@{#28517}

src/ppc/builtins-ppc.cc

@@ -1778,7 +1778,7 @@ void Builtins::Generate_ArgumentsAdaptorTrampoline(MacroAssembler* masm) {
     __ beq(&no_strong_error, cr0);
 
     // What we really care about is the required number of arguments.
-    __ lwa(r7, FieldMemOperand(r7, SharedFunctionInfo::kLengthOffset));
+    __ lwz(r7, FieldMemOperand(r7, SharedFunctionInfo::kLengthOffset));
 #if V8_TARGET_ARCH_PPC64
     // See commment near kLenghtOffset in src/objects.h
     __ srawi(r7, r7, kSmiTagSize);