[NaCl SDK] Remove pnacl.h from the sdk.
The __builtin_nacl_target_arch that it defines is not
part of the PNaCl bitcode ABI. For non-pnacl cases, it has
#error when !defined(__pnacl__) so doesn't make sense to
have there either.
It is only used in the private build of pnacl-llc.
BUG=https://code.google.com/p/nativeclient/issues/detail?id=3525
NOTRY=true
Review URL: https://codereview.chromium.org/
25039005
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225793 0039d316-1c4b-4281-b951-d872f2087c98