PR28449: runtime/transport: bump up _stp_subbuf_size to 64K

PR28449: runtime/transport: bump up _stp_subbuf_size to 64K

After commit cd48874296, it was reported that the kernel-side I/O
buffers could be exhausted more easily by the some tests in the
testsuite.  This resulted in "bufhdr corrupted ..." type messages
coming from the syscall.exp test case for example.

We bump up the $subject variable, so without "stap -s XYZ", the
default kernel->user transport will consist of 256 subbufs of 64K
each, per cpu.  (When a user does supply -s XYZ, the number gets
overridden, and the subbuf size may be quite a bit larger.)  With the
new default, the syscall.exp suite runs bufhdr-clean on an 8cpu box.