gdb/tracepoint: remove uses of sprintf

When building on macOS, I get some:

    /Users/smarchi/src/binutils-gdb/gdb/tracepoint.c:1196:4: error: 'sprintf' is deprecated: This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead. [-Werror,-Wdeprecated-declarations]
     1196 |           sprintf (end, "M-1,%s,%lX", phex_nz (m_memranges[i].start, 0),
          |           ^

Replace them with xsnprintf.

Change-Id: Id3ec76c47e5c0091fa3a028e36063b2115378e7e
Approved-By: Andrew Burgess <aburgess@redhat.com>