[libunwind] Add LIBUNWIND_BUILD_32_BITS CMake option.

[EricWF]

Summary

This option defaults to LLVM_BUILD_32_BITS but it allows it to be modified separate from the rest of LLVM.

This is intended to help fix PR27959.
(https://llvm.org/bugs/show_bug.cgi?id=27959)

[EricWF]

Abandoning in preference of D20889.

CMakeLists.txt

 #===============================================================================
 
 # Define options.
+option(LIBUNWIND_BUILD_32_BITS "Build 32 bit libunwind" ${LLVM_BUILD_32_BITS})
 option(LIBUNWIND_ENABLE_ASSERTIONS "Enable assertions independent of build mode." ON)
 option(LIBUNWIND_ENABLE_PEDANTIC "Compile with pedantic enabled." ON)
 option(LIBUNWIND_ENABLE_WERROR "Fail and stop if a warning is triggered." OFF)

   add_definitions(-D_CRT_SECURE_NO_WARNINGS)
 endif ()
 
+append_if(LIBUNWIND_COMPILE_FLAGS LIBUNWIND_BUILD_32_BITS "-m32")
+append_if(LIBUNWIND_LINK_FLAGS LIBUNWIND_BUILD_32_BITS "-m32")
 append_if(LIBUNWIND_COMPILE_FLAGS LIBUNWIND_TARGET_TRIPLE
           "-target ${LIBUNWIND_TARGET_TRIPLE}")
 append_if(LIBUNWIND_COMPILE_FLAGS LIBUNWIND_GCC_TOOLCHAIN