Add initial specification for Big Endian

[djtodoro]

GNU GCC Toolchain already supports big endian for RISC-V target. That support was merged without a change in psABI Document.
Here [0] is the initial PR for adding big endian support in LLVM project, so lets implement documentation part as well.

[0] llvm/llvm-project#146534

[rui314]

I think we need to clarify which relocations write data in big-endian order when the output is big-endian. My understanding is as follows:

R_RISCV_{32,64}
R_RISCV_ADD{16,32,64}
R_RISCV_SUB{16,32,64}
R_RISCV_SET{16,32,64}
R_RISCV_32_PCREL
R_RISCV_PLT32

[djtodoro]

I think we need to clarify which relocations write data in big-endian order when the output is big-endian. My understanding is as follows:

R_RISCV_{32,64} R_RISCV_ADD{16,32,64} R_RISCV_SUB{16,32,64} R_RISCV_SET{16,32,64} R_RISCV_32_PCREL R_RISCV_PLT32

Hi @rui314, I agree, thanks a lot for pointing that out.