Update all LLVM documentation mentioning runtimes in LLVM_ENABLE_PROJECTS

We are moving away from building the runtimes with LLVM_ENABLE_PROJECTS,
however the documentation was largely outdated. This commit updates all
the documentation I could find to use LLVM_ENABLE_RUNTIMES instead of
LLVM_ENABLE_PROJECTS for building runtimes.

Note that in the near future, libcxx, libcxxabi and libunwind will stop
supporting being built with LLVM_ENABLE_PROJECTS altogether. I don't know
what the plans are for other runtimes like libc, openmp and compiler-rt,
so I didn't make any changes to the documentation that would imply
something for those projects.

Once this lands, I will also cherry-pick this on the release/14.x branch
to make sure that LLVM's documentation is up-to-date and reflects what
we intend to support in the future.

Differential Revision: https://reviews.llvm.org/D119351
diff --git a/flang/README.md b/flang/README.md
index a8cd9c8..604049a 100644
--- a/flang/README.md
+++ b/flang/README.md
@@ -33,8 +33,8 @@
 and
 also review [how flang uses modern C++ features](docs/C++17.md).
 
-If you are interested in writing new documentation, follow 
-[markdown style guide from LLVM](https://github.com/llvm/llvm-project/blob/main/llvm/docs/MarkdownQuickstartTemplate.md).
+If you are interested in writing new documentation, follow
+[LLVM's Markdown style guide](https://github.com/llvm/llvm-project/blob/main/llvm/docs/MarkdownQuickstartTemplate.md).
 
 ## Building flang
 There are two ways to build flang. The first method is to build it at the same
@@ -55,7 +55,7 @@
 floating point numbers.  It's not needed to run the automated tests.
 
 Here's a complete set of commands to clone all of the necessary source and do
-the build.  
+the build.
 
 First clone the source:
 ```bash
@@ -79,7 +79,8 @@
   -DLLVM_TARGETS_TO_BUILD=host \
   -DCMAKE_INSTALL_PREFIX=$INSTALLDIR
   -DLLVM_LIT_ARGS=-v \
-  -DLLVM_ENABLE_PROJECTS="clang;mlir;flang;compiler-rt"
+  -DLLVM_ENABLE_PROJECTS="clang;mlir;flang" \
+  -DLLVM_ENABLE_RUNTIMES="compiler-rt"
 
 ninja
 ```