IA-64/GAS: Work around format truncation compilation errors

IA-64/GAS: Work around format truncation compilation errors

Work around compilation errors:

.../gas/config/tc-ia64.c: In function 'declare_register_set':
.../gas/config/tc-ia64.c:5375:41: error: '%u' directive output may be truncated writing between 1 and 10 bytes into a region of size 8 [-Werror=format-truncation=]
 5375 |       snprintf (name, sizeof (name), "%s%u", prefix, i);
      |                                         ^~
.../gas/config/tc-ia64.c:5375:38: note: directive argument in the range [0, 4294967294]
 5375 |       snprintf (name, sizeof (name), "%s%u", prefix, i);
      |                                      ^~~~~~
.../gas/config/tc-ia64.c:5375:7: note: 'snprintf' output 2 or more bytes (assuming 11) into a destination of size 8
 5375 |       snprintf (name, sizeof (name), "%s%u", prefix, i);
      |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

observed with GCC 10.  Pick 16 for a round 64-bit stack frame size.