Make sure any pointer being passed to RustPOSIX is 8-byte aligned

[rennergade]

We're updating our rust compiler version when we merge the domain socket branch. This fortunately/unfortunately revealed a bug where it was possible to convert pointers that arent 8-byte aligned that rust just let slide before this version (even though thats technically undefined behavior).

@RusherRG found this bug with the updated compiler when testing bind"

thread '<unnamed>' panicked at 'misaligned pointer dereference: address must be a multiple of 0x8 but is 0x64c810067d1c', src/interface/types.rs:435:61

This has the effect of giving a venue to force a trusted fault from user code. We should check all pointer alignment in NaCl before we call to RustPOSIX, and if theyre not aligned return an error.

[rennergade]

We should be able to do this easily in our pointer conversion functions since we use them for all pointer translation. Let's do a check in NaClUserToSysAddrRangeProt and NaClUserToSysAddrProt for 8-bit alignment and if not return an error.

[rennergade]

We'll need to run all tests for this to make sure it doesn't break anything.