shebangs: use `/usr/bin/env bash` instead of `/bin/bash` for BSD compat

Currently our CI machine on FreeBSD fails to build with the following
error message:

```
bytecomp/generate_runtimedef.sh runtime/caml/fail.h runtime/primitives > bytecomp/runtimedef.ml
/bin/sh: bytecomp/generate_runtimedef.sh: not found
```

This PR converts all the files with a `/bin/bash` shebang to
`/usr/bin/env bash`. Some files use `/bin/sh` instead, and they are
unchanged. It may be the case that some 'bash' files could in fact run
using 'sh', but checking this requires more work and I decided to
follow the original author's intent.

(Personally I like using bash explicitly as it gives a consistent
programming environment that I can easily test on my machine; but this
should not be at the expense of portability.)

bytecomp/generate_runtimedef.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 #**************************************************************************
 #*                                                                        *

runtime/gen_primitives.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 #**************************************************************************
 #*                                                                        *

testsuite/tests/afl-instrumentation/afltest.run

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 set -e
 

tools/check-symbol-names

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 #**************************************************************************
 #*                                                                        *

tools/ci/appveyor/appveyor_build.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #**************************************************************************
 #*                                                                        *
 #*                                 OCaml                                  *

tools/ci/travis/travis-ci.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #**************************************************************************
 #*                                                                        *
 #*                                 OCaml                                  *

tools/msvs-promote-path

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #**************************************************************************
 #*                                                                        *
 #*                                 OCaml                                  *

tools/ocaml-instr-graph

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 #**************************************************************************
 #*                                                                        *

tools/ocaml-objcopy-macosx

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 #**************************************************************************
 #*                                                                        *