[compiler-rt] Check codesign in path before using

Fixes an issue discovered with Nix where codesign is executed but Nix
cannot use codesign without faults. Use `find_program` in CMake to
ensure it is in `$PATH`.

Author: RossComputerGuy

compiler-rt/cmake/Modules/AddCompilerRT.cmake

@@ -388,7 +388,8 @@ function(add_compiler_rt_runtime name type)
         set_target_properties(${libname} PROPERTIES IMPORT_PREFIX "")
         set_target_properties(${libname} PROPERTIES IMPORT_SUFFIX ".lib")
       endif()
-      if (APPLE AND NOT CMAKE_LINKER MATCHES ".*lld.*")
+      find_program(CODESIGN codesign)
+      if (APPLE AND NOT CMAKE_LINKER MATCHES ".*lld.*" AND CODESIGN)
         # Apple's linker signs the resulting dylib with an ad-hoc code signature in
         # most situations, except:
         # 1. Versions of ld64 prior to ld64-609 in Xcode 12 predate this behavior.
@@ -403,7 +404,7 @@ function(add_compiler_rt_runtime name type)
         # argument and looking for `invalid argument "linker-signed"` in its output.
         # FIXME: Remove this once all supported toolchains support `-o linker-signed`.
         execute_process(
-          COMMAND sh -c "codesign -f -s - -o linker-signed this-does-not-exist 2>&1 | grep -q linker-signed"
+          COMMAND sh -c "${CODESIGN} -f -s - -o linker-signed this-does-not-exist 2>&1 | grep -q linker-signed"
           RESULT_VARIABLE CODESIGN_SUPPORTS_LINKER_SIGNED
         )