[CMake] Support stripping and linking output to .build-id directory

[phosek]

Summary

When installing runtimes with install-runtimes-stripped, we don't want
to just strip them, we also want to preserve the debugging information
for potential debugging. To make it possible to later find the stripped
debugging information, we want to use the .build-id layout:

https://fedoraproject.org/wiki/RolandMcGrath/BuildID#Find_files_by_build_ID

That is, for libfoo.so with build ID abcdef1234, the debugging information
will be installed into lib/debug/.build-id/ab/cdef1234. llvm-objcopy
already has support for stripping files and linking the debugging
stripped output into the right location. However, CMake doesn't support
customizing strip invocation for the *-stripped targets. So instead, we
replace CMAKE_STRIP with a custom script that invokes llvm-objcopy with
the right command line flags.

[phosek]

llvm/runtimes/CMakeLists.txt

455 ↗

(On Diff #189815)

We should probably only enable this for ELF targets since that's the only one that currently supports the .build-id scheme.

llvm/runtimes/llvm-strip-link.in

14 ↗

(On Diff #189815)

@jakehehrlich llvm-objcopy complains when you give it unrecognized file as an input, in our case it's libc++.so which is a linker script. Is there a better way to do this?

[jakehehrlich]

LGTM

llvm/runtimes/llvm-strip-link.in

14 ↗

(On Diff #189815)

No this is the best way I can think to do it. If a file starts with the elf magic it's highly unlikely to be anything else, and certainly it won't be a linker script so I think this is good.

[smeenai]

Build parts look good to me.

llvm/cmake/modules/LLVMExternalProjectUtils.cmake

38 ↗

(On Diff #189815)

I think STRIP_TOOL or STRIP_PATH or STRIP_TOOL_PATH would be more standard, but I don't care too much.

[smeenai]

LLVM already has LLVM_EXTERNALIZE_DEBUGINFO, btw ... idk if that could be easily extended for this use case.

[smeenai]

llvm/runtimes/CMakeLists.txt

455 ↗

(On Diff #189815)

Yup.

[phosek]

I've updated the patch to make the .build-id linking an option that can be set in the cache file, how does that look to you?

[smeenai]

LGTM

llvm/runtimes/CMakeLists.txt

438 ↗

(On Diff #189949)

list(APPEND)

cmake/caches/Fuchsia-stage2.cmake

if(LINUX_${target}_SYSROOT)
     set(RUNTIMES_${target}_LIBCXXABI_INSTALL_LIBRARY OFF CACHE BOOL "")
     set(RUNTIMES_${target}_LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
     set(RUNTIMES_${target}_LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")
     set(RUNTIMES_${target}_LIBCXX_ENABLE_STATIC_ABI_LIBRARY ON CACHE BOOL "")
     set(RUNTIMES_${target}_LIBCXX_ABI_VERSION 2 CACHE STRING "")
     set(RUNTIMES_${target}_SANITIZER_CXX_ABI "libc++" CACHE STRING "")
     set(RUNTIMES_${target}_SANITIZER_CXX_ABI_INTREE ON CACHE BOOL "")
     set(RUNTIMES_${target}_COMPILER_RT_USE_BUILTINS_LIBRARY ON CACHE BOOL "")
+
+    # Use .build-id link.
+    list(APPEND RUNTIME_BUILD_ID_LINK "${target}")
   endif()
 endforeach()
 
 if(FUCHSIA_SDK)
   set(FUCHSIA_aarch64_NAME arm64)
   set(FUCHSIA_x86_64_NAME x64)
   foreach(target x86_64;aarch64)
     set(FUCHSIA_${target}_COMPILER_FLAGS "-I${FUCHSIA_SDK}/pkg/fdio/include")
foreach(target x86_64;aarch64)
     set(RUNTIMES_${target}-fuchsia_LIBCXXABI_HERMETIC_STATIC_LIBRARY ON CACHE BOOL "")
     set(RUNTIMES_${target}-fuchsia_LIBCXXABI_INSTALL_STATIC_LIBRARY OFF CACHE BOOL "")
     set(RUNTIMES_${target}-fuchsia_LIBCXXABI_STATICALLY_LINK_UNWINDER_IN_SHARED_LIBRARY OFF CACHE BOOL "")
     set(RUNTIMES_${target}-fuchsia_LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
     set(RUNTIMES_${target}-fuchsia_LIBCXX_ENABLE_STATIC_ABI_LIBRARY ON CACHE BOOL "")
     set(RUNTIMES_${target}-fuchsia_LIBCXX_HERMETIC_STATIC_LIBRARY ON CACHE BOOL "")
     set(RUNTIMES_${target}-fuchsia_LIBCXX_STATICALLY_LINK_ABI_IN_SHARED_LIBRARY OFF CACHE BOOL "")
     set(RUNTIMES_${target}-fuchsia_LIBCXX_ABI_VERSION 2 CACHE STRING "")
+
+    # Use .build-id link.
+    list(APPEND RUNTIME_BUILD_ID_LINK "${target}-fuchsia")
   endforeach()
 
   set(LLVM_RUNTIME_SANITIZERS "Address" CACHE STRING "")
   set(LLVM_RUNTIME_SANITIZER_Address_TARGETS "x86_64-fuchsia;aarch64-fuchsia" CACHE STRING "")
 endif()
 
 set(LLVM_BUILTIN_TARGETS "${BUILTIN_TARGETS}" CACHE STRING "")
 set(LLVM_RUNTIME_TARGETS "${RUNTIME_TARGETS}" CACHE STRING "")
+set(LLVM_RUNTIME_BUILD_ID_LINK_TARGETS "${RUNTIME_BUILD_ID_LINK}" CACHE STRING "")
 
 # Setup toolchain.
 set(LLVM_INSTALL_TOOLCHAIN_ONLY ON CACHE BOOL "")
 set(LLVM_TOOLCHAIN_TOOLS
   dsymutil
   llc
   llvm-ar
   llvm-cov