Fix failed exec error message

Fix failed exec error message

Ref: https://sourceware.org/ml/gdb-patches/2015-07/msg00629.html

This fixes the bogus command line in the error message shown when the
SHELL environment variable points somewhere that's not something that
resembles a shell:

  $ SHELL=/nonexisting gdb /home/pedro/a.out
  (gdb) r
  Starting program: /home/pedro/a.out
 - Cannot exec /home/pedro/a.out -c exec /home/pedro/a.out .
 + Cannot exec /nonexisting -c exec /home/pedro/a.out .
  Error: No such file or directory
  During startup program exited with code 127.
  (gdb)

gdb/ChangeLog:
2015-07-24  Pedro Alves  <palves@redhat.com>

	* fork-child.c (fork_inferior): Print argv[0] instead of exec_file.