calc!_dtoi64 assertion on ARM

It seems __dtoi64 function is just not implemented, but I classify it as a bug report since it causes application crash.

Source: https://git.reactos.org/?p=reactos.git;a=blob;f=sdk/lib/crt/math/arm/__dtoi64.s;hb=ecb0c0963d19ecb3cc46582500dddbbb452ce36b

Steps to reproduce:

• You will need Windows on ARM, see https://reactos.org/wiki/ARM_Port#Debugging_steps
• If you intend to work on this, but don't have required environment, you can ask for help in the Mattermost chat
• Build calc application for ARM architecture and start it with WinDbg in WoA environment
• Switch calculator to Scientific mode (View => Scientific)
• Switch from Decimal to Hexadecimal numbers (Hex) - assertion and crash will be triggered

(d54.780): Assertion failure - code c0000420 (first chance)

calc!_dtoi64:

01361000 defc     __assertfail

0:000> kp

 # Child-SP RetAddr  Call Site

00 0058ed48 01363a56 calc!_dtoi64

01 0058ed48 01366ac0 calc!convert_real_integer(unsigned int base = 0x3ea)+0x5e [base\applications\calc\utl_ieee.c @ 122] 

Full debug log from WinDbg: calc_arm_dbg.txt