[3.7] bpo-35214: Initial clang MemorySanitizer support (GH-10479) (GH-10492)

Adds configure flags for msan and ubsan builds to make it easier to enable.
These also encode the detail that address sanitizer and memory sanitizer
should disable pymalloc.

Define MEMORY_SANITIZER when appropriate at build time and adds workarounds
to existing code to mark things as initialized where the sanitizer is otherwise unable to
determine that.  This lets our build succeed under the memory sanitizer.  not all tests
pass without sanitizer failures yet but we're in pretty good shape after this.

(cherry picked from commit 1584a00815)

Co-authored-by: Gregory P. Smith <greg@krypto.org> [Google LLC]

Python/bootstrap_hash.c

@@ -20,6 +20,10 @@
 #  endif
 #endif
 
+#ifdef MEMORY_SANITIZER
+#  include <sanitizer/msan_interface.h>
+#endif
+
 #ifdef Py_DEBUG
 int _Py_HashSecret_Initialized = 0;
 #else
@@ -143,6 +147,11 @@ py_getrandom(void *buffer, Py_ssize_t size, int blocking, int raise)
         else {
             n = syscall(SYS_getrandom, dest, n, flags);
         }
+#  ifdef MEMORY_SANITIZER
+        if (n > 0) {
+             __msan_unpoison(dest, n);
+        }
+#  endif
 #endif
 
         if (n < 0) {

Python/pymath.c

@@ -17,7 +17,9 @@ double _Py_force_double(double x)
 
 /* inline assembly for getting and setting the 387 FPU control word on
    gcc/x86 */
-
+#ifdef MEMORY_SANITIZER
+__attribute__((no_sanitize_memory))
+#endif
 unsigned short _Py_get_387controlword(void) {
     unsigned short cw;
     __asm__ __volatile__ ("fnstcw %0" : "=m" (cw));