[flang][runtime] Add FLANG_RUNTIME_NO_REAL_3 flag to build (llvm#105856)

Allow a runtime build to disable SELECTED_REAL_KIND from returning kind
3 (16-bit truncated form of 32-bit IEEE-754 floating point, a/k/a "brain
float" or bfloat16).

flang/runtime/numeric.cpp

@@ -142,6 +142,11 @@ inline RT_API_ATTRS CppTypeFor<TypeCategory::Integer, 4> SelectedRealKind(
 #else
  constexpr bool hasReal2{false};
 #endif
+#ifndef FLANG_RUNTIME_NO_REAL_3
+ constexpr bool hasReal3{true};
+#else
+ constexpr bool hasReal3{false};
+#endif
 #if defined LDBL_MANT_DIG == 64 && !defined FLANG_RUNTIME_NO_REAL_10
  constexpr bool hasReal10{true};
 #else
@@ -171,9 +176,9 @@ inline RT_API_ATTRS CppTypeFor<TypeCategory::Integer, 4> SelectedRealKind(
  }
 
  if (r <= 4) {
- kind = kind < 2 ? 2 : kind;
+ kind = kind < 2 ? (hasReal2 ? 2 : 4) : kind;
  } else if (r <= 37) {
- kind = kind < 3 ? (p == 3 ? 4 : 3) : kind;
+ kind = kind < 3 ? (hasReal3 && p != 3 ? 3 : 4) : kind;
  } else if (r <= 307) {
  kind = kind < 8 ? 8 : kind;
  } else if (hasReal10 && r <= 4931) {