gdb/riscv: use register number for 'info reg' output format choice

On RISC-V the 'info register' format for floating point registers
includes the raw register value printed as hex.  While reviewing an
upstream patch[1] I realised that this raw hex output was no longer
working, but no tests had started to fail.

Here's an example of the expected output:

  (gdb) info registers $ft0
  ft0            {float = 0, double = 0}  (raw 0x0000000000000000)

It's the '(raw 0x0000000000000000)' part that was missing once I had
applied series[1].

This commit does a few things.  First, I've extended an existing test
to check that the raw register value is printed.  I've updated the
same test to use to '-wrap' option to gdb_test_multiple.  And in
addition, in the case where the register type is a union (as in the
above example), I've relaxed the test so we no longer assume that the
field named 'float' is the first field of the union, a similar change
was part of the upstream series I was reviewing[2], so I figured I
might as well throw this in here.

The reason that patch series[1] broke the raw output, is that in
riscv_print_one_register_info, in order to identify a floating point
register, we currently inspect the register's type.  If the register
is of type float, or is a 2 or 3 element union, where each element is
a float, then we consider the register a floating point register, and
print it with the additional raw format.

In this commit I've switched to just checking the register number.  If
we're in the FP register range then we handle this as a floating point
register.

The code being changed here was introduced in commit:

  commit 270b9329b713fdc166f95dfa3a0a2f72f3a49608
  Date:   Mon Oct 22 14:10:13 2018 -0700

       RISC-V: Print FP regs as union of float types.

The commit message seems to indicate that this code was only ever
intended to identify the standard floating point registers, and I'm
not aware of any other RISC-V registers that would need printing with
the additional raw format, so I think the register number check should
be fine.

As such, there should be no user visible changes after this commit.

[1] https://inbox.sourceware.org/gdb-patches/20251105085121.1019759-1-jerry.zhangjian@sifive.com
[2] https://inbox.sourceware.org/gdb-patches/20251105085121.1019759-5-jerry.zhangjian@sifive.com

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