ARM64: Change some incorect uses of ptrdiff_t to int64_t.

src/arm64/instructions-arm64.h

Index: src/arm64/instructions-arm64.h
diff --git a/src/arm64/instructions-arm64.h b/src/arm64/instructions-arm64.h
index 9f3c20f702c5a376a888ce34d20d7d1c96f0ecff..374e2464c3d0e5b60695ee7ead9c93cfdb06fd8e 100644
--- a/src/arm64/instructions-arm64.h
+++ b/src/arm64/instructions-arm64.h
@@ -338,7 +338,7 @@ class Instruction {
   // Find the PC offset encoded in this instruction. 'this' may be a branch or
   // a PC-relative addressing instruction.
   // The offset returned is unscaled.
-  ptrdiff_t ImmPCOffset();
+  int64_t ImmPCOffset();
 
   // Find the target of this instruction. 'this' may be a branch or a
   // PC-relative addressing instruction.