disasmarm.d now can be a standalone program

[Walter Bright]

I've been using godbolt.org to figure out which #AArch64 instructions are 
generated for which operations, as I find the instruction descriptions are 
frequently baffling.

But I've been reduced to taking the godbolt 8 bit hex instructions and compare 
them against the 2000 #AArch64 instructions to get a match (yes, the mnemonic 
helps a lot, but as many mnemonics have multiple encodings, it's back to 
tedium). It's laborious because the instruction encodings do not line up on byte 
boundaries, so I have to convert it to binary and then mark out the bit fields 
by hand (dammit D needs bit fields!)

I finally realized disasmarm.d can help. I fixed it so one just types in the 8 
hex bytes on the command for the instruction, and it will disassemble it and 
provide the correct URL to the manual page for that instruction.

disasmarm.d can be compiled with:

```
dmd disasmarm.d -version=StandAlone
```

to make a standalone program and it will do a little disassembling:

```
./disasmarm fd401bff
FD 40 1B FF  ldr d31,[sp,#0x30] // 
https://www.scs.stanford.edu/~zyedidia/arm64/encodingindex.html#ldst_pos
```

It's incomplete as it doesn't yet handle all the encodings, but I work on it as 
needed.