ARM64: Change some incorect uses of ptrdiff_t to int64_t.

src/arm64/instructions-arm64.cc

Index: src/arm64/instructions-arm64.cc
diff --git a/src/arm64/instructions-arm64.cc b/src/arm64/instructions-arm64.cc
index a6ca6affae9384d794504e613fa512d870c25b8f..71094baa8788f9d995b5ae62c54f5c50f1049dfd 100644
--- a/src/arm64/instructions-arm64.cc
+++ b/src/arm64/instructions-arm64.cc
@@ -182,8 +182,8 @@ LSDataSize CalcLSPairDataSize(LoadStorePairOp op) {
 }
 
 
-ptrdiff_t Instruction::ImmPCOffset() {
-  ptrdiff_t offset;
+int64_t Instruction::ImmPCOffset() {
+  int64_t offset;
   if (IsPCRelAddressing()) {
     // PC-relative addressing. Only ADR is supported.
     offset = ImmPCRel();