Change main.scons to build 32- and 64-bit debug by default.  Also make it get the platform-specific toolchain dir without requiring NACL_TOOLCHAIN_DIR to be defined in the env.  This should allow the build to work on any of the bots.
Change main.scons to build 32- and 64-bit debug by default.  Also make it get
the platform-specific toolchain dir without requiring NACL_TOOLCHAIN_DIR to be
defined in the env.  This should allow the build to work on any of the bots.

Review URL: http://codereview.chromium.org/5138008

git-svn-id: e22621a84199d42b34715cdc089545aeedbbf23f@619 050acbb0-2703-11df-ab0a-9f3f633ae91d

sbc100/nativeclient-sdk,sbc100/nativeclient-sdk,sbc100/nativeclient-sdk,sbc100/nativeclient-sdk,sbc100/nativeclient-sdk,sbc100/nativeclient-sdk,sbc100/nativeclient-sdk