gdb/testsuite: change gdb.base/nodebug.exp to not fail with clang

gdb/testsuite: change gdb.base/nodebug.exp to not fail with clang

Clang organizes the variables differently to gcc in the original version
of this code, leading to the following differences when testing
p (int*) &dataglobal + 1

gcc:
$16 = (int *) 0x404034 <datalocal>

clang:
$16 = (int *) 0x404034 <dataglobal8>

However, since the important part of this test doesn't seem to be which
symbol is linked, but rather if GDB is correctly increasing the
address. This test was changed to actually measure address changes,
instead of assuming the ordering and naming of symbols.

Co-Authored-By: Andrew Burgess <aburgess@redhat.com>