[NaCl SDK] Always use cmd.exe for SHELL in Windows Makefiles

[NaCl SDK] Always use cmd.exe for SHELL in Windows Makefiles

If SHELL is not set then make will search PATH for sh.exe. If it is found, it
will use that instead of cmd.exe. If the sh.exe exists in a path with a space,
the command will fail to run.

BUG=none
R=sbc@chromium.org

Committed: https://crrev.com/b8c39431fc3e8631a9fc252d1c794f86e80e1235
Cr-Commit-Position: refs/heads/master@{#309450}

[Sam Clegg, 2014-12-16 20:55:07]

lgtm

[binji, 2014-12-22 19:28:30]

The CQ bit was checked by binji@chromium.org

[commit-bot: I haz the power, 2014-12-22 19:29:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/806283003/20001

[commit-bot: I haz the power, 2014-12-22 20:11:40]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2014-12-22 20:12:37]

Patchset 2 (id:??) landed as
https://crrev.com/b8c39431fc3e8631a9fc252d1c794f86e80e1235
Cr-Commit-Position: refs/heads/master@{#309450}