Change how "print/x" displays floating-point value

Currently, "print/x" will display a floating-point value by first
casting it to an integer type.  This yields weird results like:

    (gdb) print/x 1.5
    $1 = 0x1

This has confused users multiple times -- see PR gdb/16242, where
there are several dups.  I've also seen some confusion from this
internally at AdaCore.

The manual says:

    'x'
	 Regard the bits of the value as an integer, and print the integer
	 in hexadecimal.

... which seems more useful.  So, perhaps what happened is that this
was incorrectly implemented (or maybe correctly implemented and then
regressed, as there don't seem to be any tests).

This patch fixes the bug.

There was a previous discussion where we agreed to preserve the old
behavior:

    https://sourceware.org/legacy-ml/gdb-patches/2017-06/msg00314.html

However, I think it makes more sense to follow the manual.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=16242