Remove -m64 flag.
Remove -m64 flag.

On 64-bit Linux, remove the emulation mode flag, given -m64 is an
invalid emulation mode on 64-bit Linux.

Example compilation from Basho's erlang_js on Linux:

```
ld: unrecognised emulation mode: 64
Supported emulations: elf_x86_64 elf32_x86_64 elf_i386 i386linux elf_l1om elf_k1om i386pep i386pe
make[4]: *** [Linux_All_OPT.OBJ/libjs.so] Error 1
```

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