[gdb/testsuite] Prepare nodebug exec in gdb.base/cvexpr.exp

[gdb/testsuite] Prepare nodebug exec in gdb.base/cvexpr.exp

When running test-case gdb.base/cvexpr.exp with target board unix/-bad, I get:
...
gdb compile failed, gcc: error: unrecognized command line option '-bad'
ERROR: tcl error sourcing src/gdb/testsuite/gdb.base/cvexpr.exp.
ERROR: can't read "use_gdb_stub": no such variable
...

This is triggered in a part of the test that claims to require no debug
information, but uses the exec containing either dwarf or ctf.

Fix this by preparing another executable compiled with nodebug, and using
that one instead.

Also use with_test_prefix to mark the nodebug part, such that we have:
...
gdb compile failed, gcc: error: unrecognized command line option '-bad'
UNTESTED: gdb.base/cvexpr.exp: dwarf: failed to prepare
gdb compile failed, gcc: error: unrecognized command line option '-bad'
UNTESTED: gdb.base/cvexpr.exp: nodebug: failed to prepare
...

Tested on x86_64-linux.