[python/cpython] Remove "unknown_format" code path in PyFloat_Pack/Unpack*()

### ROOT CAUSE The "unknown_format" code path in PyFloat_Pack/Unpack*() functions is currently used to handle floating-point formats that are not IEEE 754. However, this code path is only necessary for certain ARM platforms and is not required for other platforms. Additionally, the "unknown_format" code path is used to detect endianness at runtime, which can be simplified by using configure macros in tests. ### CODE FIX 1. **Remove the "unknown_format" code path for unsupported platforms**: Since the "unknown_format" code path is only necessary for certain ARM platforms and is not required for other platforms, we can remove it for unsupported platforms. 2. **Simplify the endianness detection**: Instead of using the "unknown_format" code path to detect endianness at runtime, we can use configure macros in tests to detect the endianness of the platform. Here is a possible implementation of the fix:

c
#include 
#include 

#define ENDIANNESS_LITTLE 1
#define ENDIANNESS_BIG 2

static int detect_endianness(void) {
    uint32_t num = 1;
    uint8_t *ptr = (uint8_t *)#
    if (*ptr == 1) {
        return ENDIANNESS_LITTLE;
    } else {
        return ENDIANNESS_BIG;
    }
}

static void PyFloat_Pack8(double value, uint8_t *buffer) {
    if (detect_endianness() == ENDIANNESS_LITTLE) {
        // Pack little-endian format
        // ...
    } else {
        // Pack big-endian format
        // ...
    }
}

static double PyFloat_Unpack8(const uint8_t *buffer) {
    if (detect_endianness() == ENDIANNESS_LITTLE) {
        // Unpack little-endian format
        // ...
    } else {
        // Unpack big-endian format
        // ...
    }
}


In this implementation, the `detect_endianness` function is used to detect the endianness of the platform at runtime. The `PyFloat_Pack8` and `PyFloat_Unpack8` functions use the endianness detected by `detect_endianness` to pack and unpack floating-point numbers.

By removing the "unknown_format" code path for unsupported platforms and simplifying the endianness detection, we can improve the performance and reliability of the PyFloat_Pack/Unpack*() functions.