[libc] Forward LLVM_LIBC options when using a runtimes build

[jhuber6]

Summary

The `LLVM_ENABLE_RUNTIMES' mode is commonly used to build runtimes that
depend on an up-to-date version of clang. Currently, libc uses some
internal variables that are not forwarded when building in this mode.
This patch forwards the relevent arguments beginning with LLVM_LIBC to
the build when built this way.

[michaelrj]

LGTM, but I'd recommend getting review from someone more familiar with the runtimes build.

[tianshilei1992]

LGTM

llvm/runtimes/CMakeLists.txt

foreach(entry ${runtimes})
   if (${canon_name} STREQUAL "OPENMP")
     list(APPEND prefixes "LIBOMP" "LIBOMPTARGET")
   endif()
   # Many compiler-rt options start with SANITIZER_ and DARWIN_ rather than
   # COMPILER_RT_, so when compiler-rt is enabled, consider both.
   if(canon_name STREQUAL "COMPILER_RT")
     list(APPEND prefixes SANITIZER DARWIN)
   endif()
+  if(canon_name STREQUAL "LLVMLIBC")
+    list(APPEND prefixes "LLVM_LIBC")
+  endif()
 
   string(FIND ${projName} "lib" LIB_IDX)
   if(LIB_IDX EQUAL 0)
     string(SUBSTRING ${projName} 3 -1 projName)
   endif()
   list(APPEND runtime_names ${projName})
 endforeach()