x86: Replace getDoubleBits with floatToBits64.

x86: Replace getDoubleBits with floatToBits64.

The getDoubleBits function was used exactly once to find the bit
representation of a double floating point value, which is the same thing
the common floatToBits64 function does. Eliminate x86's one off version,
and use the common one instead.

Change-Id: Icb0cec5a55d81a6eacf1bb5a3c2b8f16c414d0d9
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32927
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>