bpo-42604: always set EXT_SUFFIX=${SOABI}${SHLIB_SUFFIX} when using configure (GH-23708)

Now all platforms use a value for the "EXT_SUFFIX" build variable derived
from SOABI (for instance in FreeBSD, "EXT_SUFFIX" is now ".cpython-310d.so"
instead of ".so"). Previously only Linux, Mac and VxWorks were using a value
for "EXT_SUFFIX" that included "SOABI".

Co-authored-by: Pablo Galindo <pablogsal@gmail.com>