Makefile: fix forced rebuild of LLVM

This is a small change that makes sure not to force a rebuild of LLVM
every time.

For example, I might run:

    make llvm-source
    make llvm-build ASSERT=1

And then I might make some temporary changes to LLVM to test out a patch
for example. So I run:

    make llvm-build

...and my whole build cache gets destroyed.

This commit addresses this issue by not forcing a re-run of CMake with
every `make llvm-build` invocation.

Makefile

@@ -203,7 +203,7 @@ llvm-source: $(LLVM_PROJECTDIR)/llvm
 
 # Configure LLVM.
 TINYGO_SOURCE_DIR=$(shell pwd)
-$(LLVM_BUILDDIR)/build.ninja: llvm-source
+$(LLVM_BUILDDIR)/build.ninja:
 	mkdir -p $(LLVM_BUILDDIR) && cd $(LLVM_BUILDDIR) && cmake -G Ninja $(TINYGO_SOURCE_DIR)/$(LLVM_PROJECTDIR)/llvm "-DLLVM_TARGETS_TO_BUILD=X86;ARM;AArch64;RISCV;WebAssembly" "-DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD=AVR;Xtensa" -DCMAKE_BUILD_TYPE=Release -DLIBCLANG_BUILD_STATIC=ON -DLLVM_ENABLE_TERMINFO=OFF -DLLVM_ENABLE_ZLIB=OFF -DLLVM_ENABLE_LIBEDIT=OFF -DLLVM_ENABLE_Z3_SOLVER=OFF -DLLVM_ENABLE_OCAMLDOC=OFF -DLLVM_ENABLE_LIBXML2=OFF -DLLVM_ENABLE_PROJECTS="clang;lld" -DLLVM_TOOL_CLANG_TOOLS_EXTRA_BUILD=OFF -DCLANG_ENABLE_STATIC_ANALYZER=OFF -DCLANG_ENABLE_ARCMT=OFF $(LLVM_OPTION)
 
 # Build LLVM.