compiler-rt: test: Use the host instead of the target to select %expect_crash.

When running tests in a cross-compiled build, %expect_crash is
run on the host, not the target, so we need to check the host
OS. config.host_os surprisingly refers to the target OS because it
is derived from CMAKE_SYSTEM_NAME which specifies the target. See:
https://cmake.org/cmake/help/latest/variable/CMAKE_SYSTEM_NAME.html

Another speculative fix for:
https://lab.llvm.org/buildbot/#/builders/193/builds/9070

(cherry picked from commit 968d38d)

Author: pcc

compiler-rt/test/lit.common.cfg.py

@@ -503,7 +503,7 @@ def get_ios_commands_dir():
 # Define %arch to check for architecture-dependent output.
 config.substitutions.append(("%arch", (config.host_arch)))
 
-if config.host_os == "Windows":
+if os.name == "nt":
     # FIXME: This isn't quite right. Specifically, it will succeed if the program
     # does not crash but exits with a non-zero exit code. We ought to merge
     # KillTheDoctor and not --crash to make the latter more useful and remove the