Patch

Fix options for ```CMAKE_MSVC_RUNTIME_LIBRARY```

prepare 1.2.1

Move changelog out of README.md.Merge boost.context 1.71.0

boost.context: https://www.boost.org/users/history/version_1_71_0.html
  Add support for RISC-V LP64D
  [#72](boostorg/context#72): Fix ABI violation on ppc64 ELFv2
  [#99](boostorg/context#99): cleanup of stack if creating ucontext fails