[clang] fix cmake wrong variable name from RTLIBS to RTLIB

[philass]

Summary

RTSLIBS should instread be RTLIB

 if (CLANG_DEFAULT_UNWINDLIB STREQUAL "")
   if (CLANG_DEFAULT_RTLIB STREQUAL "libgcc")
     set (CLANG_DEFAULT_UNWINDLIB "libgcc" CACHE STRING "" FORCE)
-  elseif (CLANG_DEFAULT_RTLIBS STREQUAL "libunwind") 
+  elseif (CLANG_DEFAULT_RTLIB STREQUAL "libunwind")
     set (CLANG_DEFAULT_UNWINDLIB "none" CACHE STRING "" FORCE)
   endif()
 endif()

Since CLANG_DEFAULT_RTLIBS should be CLANG_DEFAULT_RTLIB the line

set (CLANG_DEFAULT_UNWINDLIB "none" CACHE STRING "" FORCE)

will never be reached.

This addresses https://bugs.llvm.org/show_bug.cgi?id=48291

[philass]

Rerun failed build that may be fixed in

https://reviews.llvm.org/rG4827492d9fcfe4ac5df4f319e935e33bec946195

clang/CMakeLists.txt

set(CLANG_DEFAULT_RTLIB "" CACHE STRING
     "Default runtime library to use (\"libgcc\" or \"compiler-rt\", empty for platform default)" FORCE)
 endif()
 
 set(CLANG_DEFAULT_UNWINDLIB "" CACHE STRING
   "Default unwind library to use (\"none\" \"libgcc\" or \"libunwind\", empty to match runtime library.)")
 if (CLANG_DEFAULT_UNWINDLIB STREQUAL "")
   if (CLANG_DEFAULT_RTLIB STREQUAL "libgcc")
     set (CLANG_DEFAULT_UNWINDLIB "libgcc" CACHE STRING "" FORCE)
-  elseif (CLANG_DEFAULT_RTLIBS STREQUAL "libunwind")
+  elseif (CLANG_DEFAULT_RTLIB STREQUAL "libunwind")
     set (CLANG_DEFAULT_UNWINDLIB "none" CACHE STRING "" FORCE)
   endif()
 endif()
 
 if (NOT(CLANG_DEFAULT_UNWINDLIB STREQUAL "" OR
         CLANG_DEFAULT_UNWINDLIB STREQUAL "none" OR
         CLANG_DEFAULT_UNWINDLIB STREQUAL "libgcc" OR
         CLANG_DEFAULT_UNWINDLIB STREQUAL "libunwind"))