Branch relaxation: fix offset computation

On PowerPC and AArch64, the branch offset is relative to the PC of the
instruction, not the PC of the next instruction.

Also: the sign of the offset was wrong.

Author: xavierleroy

aarch64/Asmexpand.ml

@@ -440,7 +440,7 @@ module BInfo: BRANCH_INFORMATION = struct
     | _ -> 4
 
   let branch_overflow ~map pc lbl range =
-    let displ = pc + 4 - map lbl in
+    let displ = map lbl - pc in
     displ < -range || displ >= range
 
   let need_relaxation ~map pc instr =

powerpc/Asmexpand.ml

@@ -867,7 +867,7 @@ module BInfo: BRANCH_INFORMATION = struct
   let need_relaxation ~map pc instr =
     match instr with
     | Pbf(_, lbl) | Pbt(_, lbl) ->
-        let displ = pc + 4 - map lbl in
+        let displ = map lbl - pc in
         displ < -0x8000 || displ >= 0x8000
     | _ ->
         false