gh-145633: remove support for ancient ARM platforms with mixed-endian doubles

* drop DOUBLE_IS_ARM_MIXED_ENDIAN_IEEE754 macro
* use DOUBLE_IS_BIG/LITTLE_ENDIAN_IEEE754 to detect endianness of
  float/doubles
* drop "unknown_format" code path in PyFloat_Pack/Unpack*()

Author: skirpichev

Include/internal/pycore_pymath.h

@@ -182,8 +182,7 @@ extern void _Py_set_387controlword(unsigned short);
 //     (extended precision), and we don't know how to change
 //     the rounding precision.
 #if !defined(DOUBLE_IS_LITTLE_ENDIAN_IEEE754) && \
-    !defined(DOUBLE_IS_BIG_ENDIAN_IEEE754) && \
-    !defined(DOUBLE_IS_ARM_MIXED_ENDIAN_IEEE754)
+    !defined(DOUBLE_IS_BIG_ENDIAN_IEEE754)
 #  define _PY_SHORT_FLOAT_REPR 0
 #endif
 

Include/pymacconfig.h

@@ -18,7 +18,6 @@
 #undef SIZEOF_UINTPTR_T
 #undef SIZEOF_PTHREAD_T
 #undef WORDS_BIGENDIAN
-#undef DOUBLE_IS_ARM_MIXED_ENDIAN_IEEE754
 #undef DOUBLE_IS_BIG_ENDIAN_IEEE754
 #undef DOUBLE_IS_LITTLE_ENDIAN_IEEE754
 #undef HAVE_GCC_ASM_FOR_X87

Misc/NEWS.d/next/Core_and_Builtins/2026-03-08-06-18-26.gh-issue-145633.Ogu-RF.rst

@@ -0,0 +1,2 @@
+Remove support for ancient ARM platforms, using mixed-endian representation
+for doubles.  Patch by Sergey B Kirpichev.