gdb/testsuite: remove use of then keyword from library files

gdb/testsuite: remove use of then keyword from library files

The canonical form of 'if' in modern TCL is 'if {} {}'.  But there's
still a bunch of places in the testsuite where we make use of the
'then' keyword, and sometimes these get copies into new tests, which
just spreads poor practice.

This commit removes all use of the 'then' keyword from the testsuite
library files (in boards/, config/, and lib/).  Previous commits have
removed all uses of the 'then' keyword from the test script files,
this commit just cleans up the library files.

There should be no changes in what is tested after this commit.