[ubsan-minimal] Add a path for non-POSIX (and bare-metal) use of the library

[filcab]

Summary

Hook on -DKERNEL_USE (which is also used in lib/builtins) to not import
strlen and not rely on write() being implemented with the stderr on fd 2.

With this, the only requirements to use this library are:

• "Good enough" std::atomic<void*> and std::atomic<int>
• abort() being implemented
• ubsan_message(const char*) being implemented

compiler-rt/trunk/lib/ubsan_minimal/ubsan_minimal_handlers.cc

 #include "sanitizer_common/sanitizer_atomic.h"
 
 #include <stdlib.h>
 #include <stdint.h>
 #include <string.h>
 #include <unistd.h>
 
+#ifdef KERNEL_USE
+extern "C" void ubsan_message(const char *msg);
+static void message(const char *msg) { ubsan_message(msg); }
+#else
 static void message(const char *msg) {
   write(2, msg, strlen(msg));
 }
+#endif
 
 static const int kMaxCallerPcs = 20;
 static __sanitizer::atomic_uintptr_t caller_pcs[kMaxCallerPcs];
 // Number of elements in caller_pcs. A special value of kMaxCallerPcs + 1 means
 // that "too many errors" has already been reported.
 static __sanitizer::atomic_uint32_t caller_pcs_sz;
 
 __attribute__((noinline)) static bool report_this_error(void *caller_p) {