Dalvik: Add hardware support for OP_LONG_TO_DOUBLE in the interpreter

The following patch adds hardware vfp support for OP_LONG_TO_DOUBLE
in the interpreter. Previously this opcode was implemented using one of
gcc's builtin helpers.

Change-Id: I7713aa03c0b0bed8df592710dd50ab3d13d4bd9b
Signed-off-by: Serban Constantinescu <serban.constantinescu@arm.com>