linux-user/alpha: write the floating-point registers to a core dump

A guest core carried only the general-purpose registers, so a debugger
opening one reported every floating-point register as unavailable --
including the arguments of the function that crashed.

Implement HAVE_ELF_CORE_FPREGS for Alpha: define target_elf_fpregset_t
to match the kernel's layout ($f0-$f30 plus the control register in the
slot $f31 would occupy) and fill it from elf_core_copy_fpregs().

Checked with lldb on a core from a program that faults with live values
in $f16 and $f17: both read back correctly, as does the control register.

Signed-off-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Helge Deller <deller@gmx.de>
Signed-off-by: Helge Deller <deller@gmx.de>