[compiler-rt][scudo] Check for failing prctl call

A bunch of MTE tests like ./ScudoUnitTest-aarch64-Test/MemtagTest.StoreTags
can fail on aarch64-linux if the kernel doesn't support the tagged address ABI. It looks like
the call to prctl(PR_GET_TAGGED_ADDR_CTRL, 0, 0, 0, 0) can return -1, which
casted to an unsigned int and masked will return a value not equal to
PR_MTE_TCF_NONE, meaning systemDetectsMemoryTagFaultsTestOnly can return an incorrect value.

This updates the check to account for a failing prctl call.

Differential Revision: https://reviews.llvm.org/D110888

Author: PiJoules

compiler-rt/lib/scudo/standalone/memtag.h

@@ -91,9 +91,10 @@ inline bool systemDetectsMemoryTagFaultsTestOnly() {
 #ifndef PR_MTE_TCF_MASK
 #define PR_MTE_TCF_MASK (3UL << PR_MTE_TCF_SHIFT)
 #endif
-  return (static_cast<unsigned long>(
-              prctl(PR_GET_TAGGED_ADDR_CTRL, 0, 0, 0, 0)) &
-          PR_MTE_TCF_MASK) != PR_MTE_TCF_NONE;
+  int res = prctl(PR_GET_TAGGED_ADDR_CTRL, 0, 0, 0, 0);
+  if (res == -1)
+    return false;
+  return (static_cast<unsigned long>(res) & PR_MTE_TCF_MASK) != PR_MTE_TCF_NONE;
 }
 
 inline void enableSystemMemoryTaggingTestOnly() {