make endians support JS backend

[ringabout]

I want to make oids module support JS backend. But it relies on endians module. Of course in JS/browser, it is always big endianess in Nim.

And nodejs backend does need endianess I think which is wrong now.

This PR will solve targetCPU nad targetOS:
https://github.com/nim-lang/Nim/pull/14543/files#diff-5bfae5b518d4ab360a8cd26e9d7fdfef1d24dbf9f9922c6f77d8660a23792f51R121

Or I need this to make oids support JS/browser backend

when not defined(js):
  import endians
else:
  proc bigEndian32(outp, inp: pointer) {.inline.} =
    # JS backend is always big endianess.
    cast[ptr uint32](outp)[] = cast[ptr uint32](inp)[]

  proc atomicInc(memLoc: var int, x: int = 1): int {.inline, discardable.} =
    inc(memLoc, x)
    result = memLoc

[ringabout]

First make endians support JS backend.
Then rename builtin_bswap16, export that and deprecated original API.
Finally make nodejs backend support endians

[timotheecour]

can you remove this entire almost-duplicated block by making copyMem(outp, inp, k) work for js, so that there's no need to have elif defined(js): ?

=>

when useBuiltinSwap or defined(js):
...

[timotheecour]

remove 1 level of indentation:

when defined js:
 ...
elif when defined(gcc) or defined(llvm_gcc) or defined(clang):
 ...
elif ...

[ringabout]

This PR is not correct(handling pointer wrongly), close it.

[timotheecour]

Of course in JS/browser, it is always big endianess in Nim.

not true, little vs big endian, even on js, depends on architecture where code runs; but admittedly little endian will be much more common. refs timotheecour#515