Revert of Remove NACL_BUILD_ARCH_MAP (patchset #1 id:1 of https://codereview.chromium.org/1039253003/)
Revert of Remove NACL_BUILD_ARCH_MAP (patchset #1 id:1 of https://codereview.chromium.org/1039253003/)

Reason for revert:
build_nexe.py may perform cross-compliation with the "--arch flag"; it looks like one of the PNaCl FYI bots builds MIPS nexes on a x86_32 without defining the __mipsel__ flag.

I'd prefer to revert instead of hacking around this for MIPS; I'm a bit confused as to why the output from the FYI bot is using pnacl-clang and gcc and don't want to.

Original issue's description:
> Remove NACL_BUILD_ARCH_MAP.
>
> This mapping is no longer necessary now that build_config.h provides these definitions.
>
> The only exception is when NACL_BUILD_ARCH=pnacl should be defined, which build_config.h doesn't have the necessary information to detect.
>
> BUG=https://code.google.com/p/chromium/issues/detail?id=440012
>
> Committed: https://chromium.googlesource.com/native_client/src/native_client/+/1e45f86b3f0b78399d853fd187e8a3d515b1a38f

TBR=ncbray@chromium.org,jfb@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=https://code.google.com/p/chromium/issues/detail?id=440012

Review URL: https://codereview.chromium.org/1056363003

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client