Linking fails for wasm32 example in README
I'm trying to build the wasm32 example from the README, but linking fails. The error message suggests that `wasm-ld` isn't receiving the `--no-entry` flag. I'm using commit 3fbef59 of `nlvm`.

#### Reproduction

Following the read-me, I put the following files in a directory.

*`panicoverride.nim`*
```nim
proc rawoutput(s: string) = discard
proc panic(s: string) {.noreturn.} = discard
```

*`myfile.nim`*
```nim
proc adder*(v: int): int {.exportc.} =
  v + 4
```

Then I ran `nlvm`.

```bash
nlvm c --cpu:wasm32 --os:standalone --gc:none --passl:--no-entry myfile.nim
```

Even though I passed the `--passl:--no-entry` flag, linking failed with the message "entry symbol not defined."

```
Hint: used config file '[NLVM PATH]/nlvm/Nim/config/nim.cfg' [Conf]
Hint: used config file '[NLVM PATH]/nlvm/Nim/config/config.nims' [Conf]
.............................................
Hint: @["wasm-ld", "-o", "/[SOURCE PATH]/myfile.wasm", "/[HOME PATH]/.cache/nim/myfile_d/myfile.o", "--export-dynamic"] [Link]
wasm-ld: error: entry symbol not defined (pass --no-entry to suppress): _start
Error: linking failed
```