[CMake][Fuchsia] Build libc++ on top libc for baremetal (#99009)

Summary:
This is mostly a proof of concept requiring a number of workarounds, but
demonstrates that it is feasible.

Test Plan: 

Reviewers: 

Subscribers: 

Tasks: 

Tags: 


Differential Revision: https://phabricator.intern.facebook.com/D60251475

Author: petrhosek

clang/cmake/caches/Fuchsia-stage2.cmake

@@ -325,7 +325,9 @@ foreach(target armv6m-unknown-eabi;armv7m-unknown-eabi;armv8m.main-unknown-eabi)
   set(RUNTIMES_${target}_CMAKE_BUILD_TYPE MinSizeRel CACHE STRING "")
   set(RUNTIMES_${target}_CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY CACHE STRING "")
   foreach(lang C;CXX;ASM)
-    set(RUNTIMES_${target}_CMAKE_${lang}_FLAGS "--target=${target} -mthumb -Wno-atomic-alignment" CACHE STRING "")
+    # TODO: The preprocessor defines workaround various issues in libc and libc++ integration.
+    # These should be addressed and removed over time.
+    set(RUNTIMES_${target}_CMAKE_${lang}_FLAGS "--target=${target} -mthumb -Wno-atomic-alignment -D'vfprintf(stream, format, vlist)=vprintf(format, vlist)' -D'fprintf(stream, format, ...)=printf(format)' -D'timeval=struct timeval{int tv_sec; int tv_usec;}' -D'gettimeofday(tv, tz)' -D_LIBCPP_PRINT=1" CACHE STRING "")
   endforeach()
   foreach(type SHARED;MODULE;EXE)
     set(RUNTIMES_${target}_CMAKE_${type}_LINKER_FLAGS "-fuse-ld=lld" CACHE STRING "")
@@ -335,7 +337,7 @@ foreach(target armv6m-unknown-eabi;armv7m-unknown-eabi;armv8m.main-unknown-eabi)
   set(RUNTIMES_${target}_LIBCXX_ABI_VERSION 2 CACHE STRING "")
   set(RUNTIMES_${target}_LIBCXX_CXX_ABI none CACHE STRING "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")
-  set(RUNTIMES_${target}_LIBCXX_ENABLE_STATIC OFF CACHE BOOL "")
+  set(RUNTIMES_${target}_LIBCXX_ENABLE_STATIC ON CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_FILESYSTEM OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_RANDOM_DEVICE OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_LOCALIZATION OFF CACHE BOOL "")
@@ -373,7 +375,9 @@ foreach(target riscv32-unknown-elf)
   set(RUNTIMES_${target}_CMAKE_BUILD_TYPE MinSizeRel CACHE STRING "")
   set(RUNTIMES_${target}_CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY CACHE STRING "")
   foreach(lang C;CXX;ASM)
-    set(RUNTIMES_${target}_CMAKE_${lang}_FLAGS "--target=${target} -march=rv32imafc -mabi=ilp32f" CACHE STRING "")
+    # TODO: The preprocessor defines workaround various issues in libc and libc++ integration.
+    # These should be addressed and removed over time.
+    set(RUNTIMES_${target}_CMAKE_${lang}_FLAGS "--target=${target} -march=rv32imafc -mabi=ilp32f -D'vfprintf(stream, format, vlist)=vprintf(format, vlist)' -D'fprintf(stream, format, ...)=printf(format)' -D'timeval=struct timeval{int tv_sec; int tv_usec;}' -D'gettimeofday(tv, tz)' -D_LIBCPP_PRINT=1" CACHE STRING "")
   endforeach()
   foreach(type SHARED;MODULE;EXE)
     set(RUNTIMES_${target}_CMAKE_${type}_LINKER_FLAGS "-fuse-ld=lld" CACHE STRING "")
@@ -383,7 +387,7 @@ foreach(target riscv32-unknown-elf)
   set(RUNTIMES_${target}_LIBCXX_ABI_VERSION 2 CACHE STRING "")
   set(RUNTIMES_${target}_LIBCXX_CXX_ABI none CACHE STRING "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")
-  set(RUNTIMES_${target}_LIBCXX_ENABLE_STATIC OFF CACHE BOOL "")
+  set(RUNTIMES_${target}_LIBCXX_ENABLE_STATIC ON CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_FILESYSTEM OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_RANDOM_DEVICE OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_LOCALIZATION OFF CACHE BOOL "")