target: increase the maximum number of buckets

I do not know what is the reasonable number of buckets.
If there are enough buckets, the result will be accurate.

I propose increase the maximum number of buckets to 128K.
If the size of program text section is less than 256KB, every
two bytes will be occupied by one buckets.
(The minimum size of one buckets is 2 bytes in gprof implementation.)

Change-Id: If9147743cefdc36f40f21e6dc73b9b28f28c9e1e
Signed-off-by: Hsiangkai Wang <hsiangkai@gmail.com>
Reviewed-on: http://openocd.zylin.com/1608
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>