mimalloc: fix mi_atomic_yield on 32-bit ARM

Previously, `mi_atomic_yield` for 32-bit ARM:

  * Used a non-standard __ARM_ARCH__ macro to determine if the compiler
    was targeting ARMv7+ in order to emit a `yield` instruction, however
    it was often not defined so fell through to an `__armel__` branch

  * Had a logic gap in the #ifdef where an `__arm__` target would be
    expected to emit `mi_atomic_yield` but there was no condition to
    define a function for big-endian targets

Now, the standard ACLE __ARM_ARCH macro [1] [2] is used which GCC and
Clang support.

The branching logic for `__armel__` has been removed so if the target
architecture supports v7+ instructions, a `yield` is emitted, otherwise
a `nop` is emitted. This covers both big and little endian scenarios.

[1]: https://arm-software.github.io/acle/main/acle.html#arm-architecture-level
[2]: https://arm-software.github.io/acle/main/acle.html#mapping-of-object-build-attributes-to-predefines

Signed-off-by: Vincent Fazio <[email protected]>

Author: vfazio

Include/internal/mimalloc/mimalloc/atomic.h

@@ -338,7 +338,7 @@ static inline void mi_atomic_yield(void) {
   _mm_pause();
 }
 #elif (defined(__GNUC__) || defined(__clang__)) && \
-      (defined(__x86_64__) || defined(__i386__) || defined(__arm__) || defined(__armel__) || defined(__ARMEL__) || \
+      (defined(__x86_64__) || defined(__i386__) || defined(__arm__) || \
        defined(__aarch64__) || defined(__powerpc__) || defined(__ppc__) || defined(__PPC__)) || defined(__POWERPC__)
 #if defined(__x86_64__) || defined(__i386__)
 static inline void mi_atomic_yield(void) {
@@ -348,10 +348,16 @@ static inline void mi_atomic_yield(void) {
 static inline void mi_atomic_yield(void) {
   __asm__ volatile("wfe");
 }
-#elif (defined(__arm__) && __ARM_ARCH__ >= 7)
+#elif defined(__arm__)
+#if __ARM_ARCH >= 7
 static inline void mi_atomic_yield(void) {
   __asm__ volatile("yield" ::: "memory");
 }
+#else
+static inline void mi_atomic_yield(void) {
+  __asm__ volatile ("nop" ::: "memory");
+}
+#endif  // __ARM_ARCH >= 7
 #elif defined(__powerpc__) || defined(__ppc__) || defined(__PPC__) || defined(__POWERPC__)
 #ifdef __APPLE__
 static inline void mi_atomic_yield(void) {
@@ -362,10 +368,6 @@ static inline void mi_atomic_yield(void) {
   __asm__ __volatile__ ("or 27,27,27" ::: "memory");
 }
 #endif
-#elif defined(__armel__) || defined(__ARMEL__)
-static inline void mi_atomic_yield(void) {
-  __asm__ volatile ("nop" ::: "memory");
-}
 #endif
 #elif defined(__sun)
 // Fallback for other archs