[compiler-rt] [compiler-rt] Add big endian support to __aeabi_(idivmod|uidivmod|uldivmod) (PR #126277)

[Peter Smith via llvm-commits]

================
@@ -40,7 +40,12 @@ DEFINE_COMPILERRT_FUNCTION(__aeabi_idivmod)
         mov     r1, r3
 #endif
         bl      SYMBOL_NAME(__divmodsi4)
+#if _YUGA_BIG_ENDIAN
----------------
smithp35 wrote:

IIUC we're changing the order so that r1 contains the quotient and r0 contains the remainder for big-endian.

My interpretation of the ABI of `__aeabi_idivmod` https://github.com/ARM-software/abi-aa/blob/main/rtabi32/rtabi32.rst#531integer-3232--32-division-functions quoting:
```
typedef struct { int quot; int rem; } idiv_return;
__value_in_regs idiv_return __aeabi_idivmod(int numerator, int denominator);
``` 
As I understand it we wouldn't endian reverse a composite type. From https://github.com/ARM-software/abi-aa/blob/main/aapcs32/aapcs32.rst#65parameter-passing
```
When a Composite Type argument is assigned to core registers (either fully or partially), the behavior is as if the argument had been stored to memory at a word-aligned (4-byte) address and then loaded into consecutive registers using a suitable load-multiple instruction.
```

Can you elaborate on why this change is necessary to fix the problem?


https://github.com/llvm/llvm-project/pull/126277