Dalvik: Add hardware vfp support for OP_LONG_TO_DOUBLE in JIT

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

Change-Id: I3c88b2a527dea99dcefdb34be6695e75993da73e
Signed-off-by: Serban Constantinescu <serban.constantinescu@arm.com>