[arm] d0..d15 are 64-bit each, not 32-bit

[arm] d0..d15 are 64-bit each, not 32-bit

When unwinding the stack, the floating point registers d0 to d15
need to be handled as double words, not words.

Only the first 8 registers have been confirmed fixed with this patch
on a STM32F407-DISC0 board, but the upper 8 registers on Cortex-M33
should be handled in the same way.

The test consisted of running a program compiled with float-abi=hard.
In the main function, a function taking a double as an argument was
called. After the function call, a hardware timer was used to
trigger an interrupt.

In the debug session, a breakpoint was set in the function called
from main to verify the content of the registers using "info float"
and another breakpoint in the interrupt handler was used to check
the same registers using "info float" on frame 2 (the frame just
before the dummy frame created for the signal handler in gdb).

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@st.com>
Signed-off-by: Yvan Roux <yvan.roux@foss.st.com>