[gdb/testsuite] Make is_64_target more robust

[gdb/testsuite] Make is_64_target more robust

I ran test-case gdb.dwarf2/opt-out-not-implptr.exp with make-check-all.sh, and
with target board dwarf64 ran into:
...
FAIL: gdb.dwarf2/opt-out-not-implptr.exp: print noptr
...
due to is_target_64 failing because of:
...
builtin_spawn -ignore SIGHUP gcc -fno-stack-protector \
  -fdiagnostics-color=never -w -c -gdwarf64 -g -o is_64_target.o \
  is_64_target.c^M
gcc: error: '-gdwarf64' is ambiguous; use '-gdwarf-64' for DWARF version or \
  '-gdwarf -g64' for debug level^M
compiler exited with status 1
...

The FAIL is the same FAIL I run into with target board unix/-m32: is_target_64
fails for both cases.

The reason that is_target_64 is failing for target board dwarf64, is because
of using system compiler 7.5.0 which doesn't support -gdwarf64.

Fix this by making is_target_64 use nodebug instead of debug for compilation.

Tested on x86_64-linux.