[wasm] Fix linkage for PPC.

Floating point param and return registers should be within the
compiler's allocatable set.

TEST=cctest/test-run-wasm-js/Run_Float64Add_jswrapped
R=titzer@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1714223002

Cr-Commit-Position: refs/heads/master@{#34194}

src/compiler/wasm-linkage.cc

@@ -122,8 +122,8 @@ LinkageLocation stackloc(int i) {
 // ===========================================================================
 #define GP_PARAM_REGISTERS r3, r4, r5, r6, r7, r8, r9, r10
 #define GP_RETURN_REGISTERS r3, r4
-#define FP_PARAM_REGISTERS d0, d1, d2, d3, d4, d5, d6, d7
-#define FP_RETURN_REGISTERS d0, d1
+#define FP_PARAM_REGISTERS d1, d2, d3, d4, d5, d6, d7, d8
+#define FP_RETURN_REGISTERS d1, d2
 
 #else
 // ===========================================================================