Use /bin/sh as shebang in gdb/make-init-c

Use /bin/sh as shebang in gdb/make-init-c

While testing the NixOS[1] packaging for gdb-11.0.90.tar.xz, I got the
following error:

  [...]
  CXX    aarch32-tdep.o
  CXX    gdb.o
  GEN    init.c
  /nix/store/26a78ync552m8j4sbjavhvkmnqir8c9y-bash-4.4-p23/bin/bash: ./make-init-c: /usr/bin/env: bad interpreter: No such file or directory
  make[2]: *** [Makefile:1866: stamp-init] Error 126
  make[2]: *** Waiting for unfinished jobs....
  make[2]: Leaving directory '/build/gdb-11.0.90/gdb'
  make[1]: *** [Makefile:9814: all-gdb] Error 2
  make[1]: Leaving directory '/build/gdb-11.0.90'
  make: *** [Makefile:903: all] Error 2
  builder for '/nix/store/xs8my3rrc3l4kdlbpx0azh6q0v0jxphr-gdb-gdb-11.0.90.drv' failed with exit code 2
  error: build of '/nix/store/xs8my3rrc3l4kdlbpx0azh6q0v0jxphr-gdb-gdb-11.0.90.drv' failed

In the nix build environment, /usr/bin/env is not present, only /bin/sh
is.  This patch makes sure that gdb/make-init-c uses '/bin/sh' as
interpreter as this is the only one available on this platform.

I do not think this change will cause regressions on any other
configuration.

[1] https://nixos.org/

gdb/Changelog

	* make-init-c: Use /bin/sh as shebang.