Subzero: Remove the need for a separate NaCl SDK installation.

Now it assumes tests are being run from within the Subzero portion of the native_client tree, and sets up PATH to find PNaCl tools relative to there.

It may be necessary to do a one-time setup to be able to build pexes:

    pnacl/build.sh sdk newlib

or the equivalent scons commands.

If the tool chain is updated, propagate the changes via:

    toolchain_build/toolchain_build_pnacl.py llvm_i686_linux --install=toolchain/linux_x86/pnacl_newlib

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/525603002
diff --git a/Makefile.standalone b/Makefile.standalone
index 7740a40..bac279a 100644
--- a/Makefile.standalone
+++ b/Makefile.standalone
@@ -90,7 +90,7 @@
 	$(LLVM_SRC_PATH)/utils/lit/lit.py -sv tests_lit
 
 check: check-lit
-	(cd crosstest; LLVM_BIN_PATH=$(LLVM_BIN_PATH) ./runtests.sh)
+	(cd crosstest; ./runtests.sh)
 
 # TODO: Fix the use of wildcards.
 # Assumes clang-format is within $PATH.