[gdb/testsuite] Restructure gdb.base/backtrace-through-cu-nodebug.exp

I noticed that the test names in test-case
gdb.base/backtrace-through-cu-nodebug.exp are a bit inconsistent:
...
PASS: $exp: no-cfi: maint frame-unwinder disable ARCH
PASS: $exp: verify no-filters unwind fail without CFI
PASS: $exp: maint flush register-cache
PASS: $exp: verify unwind fail without CFI
PASS: $exp: cfi: maint frame-unwinder disable ARCH
PASS: $exp: Verify unwinding works based only on CFI information
...

There's both a no-cfi prefix, and "without CFI".

Fix this by using proc_with_prefix, getting us a consistent prefix:
...
PASS: $exp: no-cfi: maint frame-unwinder disable ARCH
PASS: $exp: no-cfi: verify no-filters unwind fail
PASS: $exp: no-cfi: maint flush register-cache
PASS: $exp: no-cfi: verify unwind fail
PASS: $exp: cfi: maint frame-unwinder disable ARCH
PASS: $exp: cfi: Verify unwinding works
...

While we're at it, use multi_line to make a regexp more readable.

Tested on aarch64-linux.

Reviewed-By: Keith Seitz <keiths@redhat.com>