[gdb/testsuite] Fix gdb.arch/i386-mpx.exp

Since commit c4a3dbaf ("Expose current 'print' settings to Python") we
have:
...
(gdb) print /x $bnd0 = {0x10, 0x20}^M
$22 = {lbound = 0x10, ubound = 0x20} : size 0x11^M
(gdb) FAIL: gdb.arch/i386-mpx.exp: verify size for bnd0
...

The regexp in the test-case expects "size 17".

Fix this by updating the regexp.

Tested on x86_64-linux.