Remove -Werror from pnacl build, due to default switch error in subzero.

When compiling using toolchain_build_pnacl.py, we get errors of form:

  Don't use default labels in fully covered switches over enumerations

I tried different combinations of -Wno-covered-switch-default and
-Wno-error=covered-switch-default, but was not able to stop this
error from being generated. Hence, taking the simplier route of
removing -Werror from Makefile.

(see www.llvm.org/docs/CodingStandards.html for more details)

BUG=None
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/686103006
diff --git a/Makefile b/Makefile
index b8934e4..47a48fd 100644
--- a/Makefile
+++ b/Makefile
@@ -12,7 +12,7 @@
 # Include LLVM common makefile.
 include $(LEVEL)/Makefile.common
 
-CXX.Flags += -std=c++11 -Wextra -Werror -Wno-error=unused-parameter
+CXX.Flags += -std=c++11
 
 CPP.Defines += -DALLOW_TEXT_ASM=1 -DALLOW_DUMP=1 -DALLOW_LLVM_CL=1 \
                -DALLOW_LLVM_IR=1 -DALLOW_LLVM_IR_AS_INPUT=1