Merging r310796:

Merging r310796:
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r310796 | asb | 2017-08-13 11:49:33 -0700 (Sun, 13 Aug 2017) | 16 lines

Remove RISCV from LLVM_ALL_TARGETS in CMakeLists.txt

It was mistakenly added to that list in D23560 (committed in rL285712). RISCV
is an experimental backend and should never have been in that list, I
mistakenly interpreted LLVM_ALL_TARGETS as a list of all targets rather than
targets to build by default. Unfortunately, because of this the RISCV backend
has been building by default when it shouldn't be.

This commet adds a description comment, which should help to avoid such
mistakes in the future.

See my message to llvm-dev for more information and analysis
<http://lists.llvm.org/pipermail/llvm-dev/2017-August/116347.html>.

Differential Revision: https://reviews.llvm.org/D36538

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_50@310900 91177308-0d34-0410-b5e6-96231b3b80d8