Add missing call to `__sanitizer::InitializePlatformEarly()` in UBSan's standalone init.

[delcypher]

Summary

While working on rdar://problem/62083617 I noticed this call was
missing.

This is a no-op for all platforms except Darwin. For Darwin this
means the use_xnu_fast_mmap flag is initialized as it was intended
when using UBSan in standalone mode.

[yln]

So technically this is not necessary (standalone UBSan does not mmap memory), but good for consistency in case it stops being a noop?

[vsk]

In D78532#1994909, @yln wrote:

So technically this is not necessary (standalone UBSan does not mmap memory), but good for consistency in case it stops being a noop?

That sounds right to me.

compiler-rt/lib/ubsan/ubsan_init.cpp

 static void CommonInit() {
   InitializeSuppressions();
 }
 
 static void CommonStandaloneInit() {
   SanitizerToolName = GetSanititizerToolName();
   CacheBinaryName();
   InitializeFlags();
+  __sanitizer::InitializePlatformEarly();
   __sanitizer_set_report_path(common_flags()->log_path);
   AndroidLogInit();
   InitializeCoverage(common_flags()->coverage, common_flags()->coverage_dir);
   CommonInit();
   Symbolizer::LateInitialize();
 }
 
 void __ubsan::InitAsStandalone() {