[CMake][Fuchsia] Install llvm-elfabi

The canonical Fuchsia toolchain configuration installs llvm-elfabi.

Reviewed By: haowei

Differential Revision: https://reviews.llvm.org/D92444

clang/cmake/caches/Fuchsia-stage2.cmake

@@ -245,6 +245,7 @@ set(LLVM_TOOLCHAIN_TOOLS
   llvm-dlltool
   llvm-dwarfdump
   llvm-dwp
+  llvm-elfabi
   llvm-gsymutil
   llvm-lib
   llvm-mt