[CMake][Fuchsia] Install libc++ for baremetal targets (#99372)

Summary: We already build the library and want to install it also.

Test Plan: 

Reviewers: 

Subscribers: 

Tasks: 

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Differential Revision: https://phabricator.intern.facebook.com/D60250960

clang/cmake/caches/Fuchsia-stage2.cmake

@@ -348,7 +348,6 @@ foreach(target armv6m-unknown-eabi;armv7m-unknown-eabi;armv8m.main-unknown-eabi)
   set(RUNTIMES_${target}_LIBCXX_ENABLE_RTTI OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_THREADS OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_MONOTONIC_CLOCK OFF CACHE BOOL "")
-  set(RUNTIMES_${target}_LIBCXX_INSTALL_LIBRARY OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
   set(RUNTIMES_${target}_LLVM_INCLUDE_TESTS OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LLVM_ENABLE_ASSERTIONS OFF CACHE BOOL "")
@@ -399,7 +398,6 @@ foreach(target riscv32-unknown-elf)
   set(RUNTIMES_${target}_LIBCXX_ENABLE_RTTI OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_THREADS OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_MONOTONIC_CLOCK OFF CACHE BOOL "")
-  set(RUNTIMES_${target}_LIBCXX_INSTALL_LIBRARY OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
   set(RUNTIMES_${target}_LLVM_INCLUDE_TESTS OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LLVM_ENABLE_ASSERTIONS OFF CACHE BOOL "")