[gdb/testsuite] Replace remaining -gt with -gctf

With test-cases gdb.base/cvexpr.exp and gdb.base/whatis.exp I run into:
...
gdb compile failed, gcc: error: unrecognized debug output level 't'
...

This is due to using additional_flags=-gt.

Commit ffb3f587933 ("CTF: multi-CU and archive support") replaced
additional_flags=-gt with additional_flags=-gctf in gdb.ctf/*.exp and
gdb.base/ctf-*.exp.

Do the same in these two test-cases.

Tested on x86_64-linux.