[gdb/testsuite] Fix gdb.base/reggroups.exp with check-read1

[gdb/testsuite] Fix gdb.base/reggroups.exp with check-read1

On aarch64-linux, with make target check-read1, I run into:
...
(gdb) info reg vector^M
  ...
d19            {f = 0x0, u = 0x0, s = 0x0} {f =FAIL: gdb.base/reggroups.exp: fetch reggroup regs vector (timeout)
 0, u = 0, s = 0}^M
...

The problem is that while (as documented) the corresponding gdb_test_multiple
doesn't work for vector registers, it doesn't skip them either.  This causes
the timeout, and it also causes the registers after a vector register not to
be found.

Fix this by using -lbl style matching.

Make which reggroups and registers are found more explicit using verbose -log,
which makes us notice that regnames with underscores are skipped, so fix that
as well.

While we're at it, this:
...
set invalid_register_re "Invalid register .*"
...
and this:
...
       -re $invalid_register_re {
           fail "$test (unexpected invalid register response)"
       }
...
means that the prompt may or may not be consumed.  Fix this by limiting the
regexp to one line, and using exp_continue.

While we're at it, improve readability of the complex regexp matching a single
register by factoring out regexps.

Tested on aarch64-linux and x86_64-linux.