Cast RLIM_INFINITY to rlim_t to fix 64-bit Solaris 10 build

Cast RLIM_INFINITY to rlim_t to fix 64-bit Solaris 10 build

gdb doesn't currently build on 64-bit Solaris 10:

/vol/src/gnu/gdb/hg/master/local/gdb/utils.c: In function ‘void dump_core()’:
/vol/src/gnu/gdb/hg/master/local/gdb/utils.c:223:55: error: narrowing conversion
 of ‘-3’ from ‘long int’ to ‘rlim_t’ {aka ‘long unsigned int’} inside {
} [-Wnarrowing]
   struct rlimit rlim = { RLIM_INFINITY, RLIM_INFINITY };
                                                       ^
/vol/src/gnu/gdb/hg/master/local/gdb/utils.c:223:55: error: narrowing conversion
 of ‘-3’ from ‘long int’ to ‘rlim_t’ {aka ‘long unsigned int’} inside {
} [-Wnarrowing]

This was introduced by

2018-08-27  Tom Tromey  <tom@tromey.com>

	PR build/23087:
	* configure: Rebuild.
	* warning.m4 (AM_GDB_WARNINGS): Remove -Wno-narrowing.

and can be fixed by the following patch.

Solaris 11 isn't affected because there <sys/resource.h> has

#define	RLIM_INFINITY	((rlim_t)-3l)

instead of

#define RLIM_INFINITY   (-3l)

on Solaris 10.

Tested on amd64-pc-solaris2.10 and amd64-pc-solaris2.11.

	* utils.c (dump_core) [HAVE_SETRLIMIT]: Cast RLIM_INFINITY to
	rlim_t.