Also fixes:

MDEV-27768 MDEV-25440: Assertion `(cs->state & 0x20000) == 0' failed in my_strnncollsp_nchars_generic_8bit

The "strnncollsp_nchars" virtual function pointer for tis620_thai_nopad_ci
was incorrectly initialized to a generic function
my_strnncollsp_nchars_generic_8bit(), which crashed on assert.

Implementing a tis620 specific function version.