Endianness/Byte-order detection on build machine

[Hyiker]

Is your feature request related to a problem? Please describe.

I'm trying to put some byte-order related functionality in my cpp code, it works like(from UE5 source code):

struct {
#if PLATFORM_LITTLE_ENDIAN
			uint32	Mantissa : 23;
			uint32	Exponent : 8;
			uint32	Sign : 1;			
#else
			uint32	Sign : 1;
			uint32	Exponent : 8;
			uint32	Mantissa : 23;			
#endif
}

c/cpp lacks standard macros for this, I noticed there was once an issue mentioned this: #514 .However, __BYTE_ORDER__ is not a portable way to do this, for some compilers such as MSVC doesn't have such definition.

CMake provides TestBigEndian snippet(supserseded by CMAKE_<LANG>_BYTE_ORDER since 3.20), supporting c/c++/cuda/objc/objcxx. Providing resembling API greatly empowers languages such as cpp lack the ability of compile-time byte order determination.

Describe the solution you'd like

CMake uses TestEndianess.c.in for this functionality, and we can refer to TestEndianess.c.in to do such test.
I suggest run this test in configuration stage( xmake f ), and provide an variable var.$(byteorder) describing the endianness. To reduce complexity, the value should be either big_endian or little_endian, ignoring other conditions.

Describe alternatives you've considered

No response

Additional context

No response

[waruqi]

you can refer this patch. #4349

then open a pr to add check_bigendian

[waruqi]

Thanks!