Use the new file location for proc.ml (#140)

Fix line directives at the beginning of proc.ml files in the
backend directory to refer to their new source location (instead of
the original asmcomp directory).
Add missing directives for arm, arm64 power, s390x, riscv.

backend/amd64/proc.ml

@@ -1,4 +1,4 @@
-# 2 "asmcomp/amd64/proc.ml"
+# 2 "backend/amd64/proc.ml"
 (**************************************************************************)
 (*                                                                        *)
 (*                                 OCaml                                  *)

backend/arm/proc.ml

@@ -1,3 +1,4 @@
+# 2 "backend/arm/proc.ml"
 (**************************************************************************)
 (*                                                                        *)
 (*                                 OCaml                                  *)

backend/arm64/proc.ml

@@ -1,3 +1,4 @@
+# 2 "backend/arm64/proc.ml"
 (**************************************************************************)
 (*                                                                        *)
 (*                                 OCaml                                  *)

backend/i386/proc.ml

@@ -1,4 +1,4 @@
-# 2 "asmcomp/i386/proc.ml"
+# 2 "backend/i386/proc.ml"
 (**************************************************************************)
 (*                                                                        *)
 (*                                 OCaml                                  *)

backend/power/proc.ml

@@ -1,3 +1,4 @@
+# 2 "backend/power/proc.ml"
 (**************************************************************************)
 (*                                                                        *)
 (*                                 OCaml                                  *)

backend/riscv/proc.ml

@@ -1,3 +1,4 @@
+# 2 "backend/riscv/proc.ml"
 (**************************************************************************)
 (*                                                                        *)
 (*                                 OCaml                                  *)

backend/s390x/proc.ml

@@ -1,3 +1,4 @@
+# 2 "backend/s390x/proc.ml"
 (**************************************************************************)
 (*                                                                        *)
 (*                                 OCaml                                  *)