Tidy: Make NACL_x86/arm #defines more robust against NACL_BUILD_ARCH not being defined
Tidy: Make NACL_x86/arm #defines more robust against NACL_BUILD_ARCH not being defined

We discovered that header files were silently assuming x86 if
NACL_BUILD_ARCH was not defined.  For PNaCl, we don't want to
#define NACL_BUILD_ARCH.

BUG=none
TEST=trybots

Review URL: http://codereview.chromium.org/8883036

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@7405 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,sbc100/native_client,nacl-webkit/native_client,nacl-webkit/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,nacl-webkit/native_client,nacl-webkit/native_client