gdb: make signal_catch_counts a static array

I noticed that signal_catch_counts is a dynamically-allocated array of
constant size, allocated at startup an never freed.  This might as well
be a statically-allocated array.

gdb/ChangeLog:

	* break-catch-sig.c (signal_catch_counts): Make a static arrray.
	(_initialize_break_catch_sig): Don't allocate array.

Change-Id: I220321df5ad6c1d2664ec9d483eea2dc1c979afe