Revert of Remove NACL_BUILD_ARCH_MAP

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

Committed: https://chromium.googlesource.com/native_client/src/native_client/+/9b893060e6f45d52b665cf7160c073eec37edda5

[teravest, 2015-04-06 15:13:36]

Created Revert of Remove NACL_BUILD_ARCH_MAP

[commit-bot: I haz the power, 2015-04-06 15:13:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1056363003/1

[commit-bot: I haz the power, 2015-04-06 15:13:47]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-04-06 15:13:53]

No LGTM from a valid reviewer yet. Only full committers are accepted.
Even if an LGTM may have been provided, it was from a non-committer,
_not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[Nick Bray (chromium), 2015-04-06 17:28:12]

LGTM

[teravest, 2015-04-06 17:31:31]

The CQ bit was checked by teravest@chromium.org

[commit-bot: I haz the power, 2015-04-06 17:31:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1056363003/1

[commit-bot: I haz the power, 2015-04-06 17:31:56]

Committed patchset #1 (id:1) as
https://chromium.googlesource.com/native_client/src/native_client/+/9b893060e...