Wasm32: memcpy import
When compiling a king_of_the_hill.nim example in https://github.com/status-im/nimplay/tree/af68eb68482c4114fb507dd41cd3d2c9bf3f670e
`memcpy` will imported, which is undesired (memcpy should be included in the file).
```
$ wasm2wat examples/king_of_the_hill.wasm | grep memcpy
  (import "env" "memcpy" (func (;3;) (type 2)))
```
Which the equivalent clang + nim does not do, I suspect this is related to the clang flag `-nostdlib` which I am unable to set with _nlvm_.

The nim + clang compiler uses the following options:
```
--os:standalone
--cpu:i386
--cc:clang
--gc:none
--nomain
--target=wasm32-unknown-unknown-wasm
--clang.options.linker:"-nostdlib -Wl,--no-entry,--allow-undefined,--strip-all"
```
NLVM uses:
```
-d:release 
--nlvm.target=wasm32 
--gc:none
 -l:--compress-relocations
 -l:--no-entry
 -l:--allow-undefined
 -l:--strip-all
 --noMain
```