PPC: [codegen] Add FP returns to code stub linkage

Port ab547021

Original Commit Message:

    - Adds kFPReturnRegister0 for all platforms.
    - Reworks linkage.cc to assign return registers to the proper
      register type, using the new FP return register.

R=bbudge@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=
LOG=N

Change-Id: I549f408ff1d0eed604c7bd38518f31dadb5d01f6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2168356Reviewed-by: Junliang Yan <jyan@ca.ibm.com>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Milad Farazmand <miladfar@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#67410}

src/codegen/ppc/register-ppc.h

@@ -306,7 +306,7 @@ constexpr Register kRuntimeCallArgvRegister = r5;
 constexpr Register kWasmInstanceRegister = r10;
 constexpr Register kWasmCompileLazyFuncIndexRegister = r15;
 
-constexpr DoubleRegister kFPReturnRegister0 = d0;
+constexpr DoubleRegister kFPReturnRegister0 = d1;
 
 }  // namespace internal
 }  // namespace v8