gdb: add make-init-c script

gdb: add make-init-c script

I would like to modify how the init.c file is generated (its content).
But as it is, a shell script with multiple sed invocations in a Makefile
target, it's not very maintainable.  Replace that with a shell script
that does the same, but in a more readable way.

The Makefile rule uses the "-" prefix in front of the for loop, I
presume to ignore any error coming from the fact that xml-builtin.c and
cp-name-parser.c are not found in the srcdir (they are generated source
files).  I prefer not to blindly ignore errors, so filter these files
out of INIT_FILES instead (we already filter out other files).

There are no expected meaningful changes to the generated init.c file.
Just the _initialize_all_file declaration that is moved down and "void"
in parenthesis that is removed.

The new regular expression is a bit tighter than the existing one, it
requires the init function to be followed by exactly ` ()`.  Update
bpf-tdep.c accordingly.

gdb/ChangeLog:

	* Makefile.in (INIT_FILES_FILTER_OUT): New.
	(INIT_FILES): Use INIT_FILES_FILTER_OUT.
	(stamp-init): Use make-init-c.
	* bpf-tdep.c (_initialize_bpf_tdep): Remove "void".
	* silent-rules.mk (ECHO_INIT_C): Change.
	* make-init-c: New file.

Change-Id: I6d6b12cbccf24ab79d1219bff05df01624c684f9