Add LLVM_EXPERIMENTAL_TARGETS and default it to WebAssembly

This provides feature-parity between Julia-built LLVM and
BinaryBuilder-built LLVM, allowing us to vendor dylibs (such as
`libclang`) to users through BinaryProvider without causing segfaults
within Julia due to mismatched features.  While the `WebAssembly`
backend changes enough within the built LLVM to make it so that we
cannot simply take `libclang` from a build that has WASM support and use
it within the offcial Julia library, it appears that it does not change
the codepaths that Julia itself uses enough to cause any issues (at
least in the testing that we have done).

In LLVM 8, the WASM backend is expected to be promoted out of
experimental.

deps/llvm.mk

@@ -52,6 +52,7 @@ endif # LLVM_VER != svn
 
 # Figure out which targets to build
 LLVM_TARGETS := host;NVPTX;AMDGPU
+LLVM_EXPERIMENTAL_TARGETS := WebAssembly
 
 LLVM_CFLAGS :=
 LLVM_CXXFLAGS :=
@@ -64,7 +65,7 @@ LLVM_CFLAGS += $(CFLAGS)
 LLVM_CXXFLAGS += $(CXXFLAGS)
 LLVM_CPPFLAGS += $(CPPFLAGS)
 LLVM_LDFLAGS += $(LDFLAGS)
-LLVM_CMAKE += -DLLVM_TARGETS_TO_BUILD:STRING="$(LLVM_TARGETS)" -DCMAKE_BUILD_TYPE="$(LLVM_CMAKE_BUILDTYPE)"
+LLVM_CMAKE += -DLLVM_TARGETS_TO_BUILD:STRING="$(LLVM_TARGETS)" -DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD="$(LLVM_EXPERIMENTAL_TARGETS)" -DCMAKE_BUILD_TYPE="$(LLVM_CMAKE_BUILDTYPE)"
 LLVM_CMAKE += -DLLVM_ENABLE_ZLIB=OFF -DLLVM_ENABLE_LIBXML2=OFF
 ifeq ($(USE_POLLY_ACC),1)
 LLVM_CMAKE += -DPOLLY_ENABLE_GPGPU_CODEGEN=ON