[cmake] Handle iOS, watchOS and tvOS when finding compiler-rt on Apple

[ldionne]

Summary

This patch uses CMAKE_OSX_SYSROOT, which should contain the SDK path
that we're building against on Apple platforms, to determine which
platform we are compiling for and set the compiler-rt suffix accordingly.

[ldionne]

Rebase onto main to poke CI.

[ldionne]

Handle simulators while we're at it.

[phosek]

LGTM

cmake/Modules/HandleCompilerRT.cmake

function(cache_compiler_rt_library err_flag name target library_file)
   endif()
 endfunction()
 
 function(get_component_name name variable)
   if(APPLE)
     if(NOT name MATCHES "builtins.*")
       set(component_name "${name}_")
     endif()
-    # TODO: Support ios, tvos and watchos as well.
+    if (CMAKE_OSX_SYSROOT MATCHES ".+MacOSX.+")
       set(component_name "${component_name}osx")
+
+    elseif (CMAKE_OSX_SYSROOT MATCHES ".+iPhoneOS.+")
+      set(component_name "${component_name}ios")
+    elseif (CMAKE_OSX_SYSROOT MATCHES ".+iPhoneSimulator.+")
+      set(component_name "${component_name}iossim")
+
+      elseif (CMAKE_OSX_SYSROOT MATCHES ".+AppleTVOS.+")
+      set(component_name "${component_name}tvos")
+    elseif (CMAKE_OSX_SYSROOT MATCHES ".+AppleTVSimulator.+")
+      set(component_name "${component_name}tvossim")
+
+    elseif (CMAKE_OSX_SYSROOT MATCHES ".+WatchOS.+")
+      set(component_name "${component_name}watchos")
+    elseif (CMAKE_OSX_SYSROOT MATCHES ".+WatchSimulator.+")
+      set(component_name "${component_name}watchossim")
+    else()
+      message(FATAL_ERROR "Unknown Apple SDK ${CMAKE_OSX_SYSROOT}, we don't know which compiler-rt library suffix to use.")
+    endif()
   else()
     set(component_name "${name}")
   endif()
   set(${variable} "${component_name}" PARENT_SCOPE)
 endfunction()
 
 # Find the path to compiler-rt library `name` (e.g. "builtins") for the
 # specified `TARGET` (e.g. "x86_64-linux-gnu") and return it in `variable`.