Add big-endian build docs

Docs/Building-BE.md

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+# Building Big Endian ASTC Encoder
+
+**NOTE:** The current code is _NOT_ BE compatible, but we plan to fix this!
+
+We don't officially support a big-endian compressor build, but ensuring the
+code is BE-compatible is good practice and some of the open source
+distributions still support BE platforms.
+
+Even though Arm64 can run in a BE mode, it's now very rare in practice. It's no
+longer supported out of the box in the latest Arm upstream compiler releases,
+and getting hold of a sysroot is increasingly painful. To test BE builds I
+therefore cross-compile Linux builds for MIPS64 and use `qemu-user` to run
+them. This doesn't use a real sysroot, so everything must be compiled with
+`-static` linkage.
+
+## Host software
+
+Install the following host software:
+
+```bash
+# Compiler
+sudo apt-get install g++-mips64-linux-gnuabi64
+
+# Multi-arch libraries
+sudo apt-get install g++-multilib-mips64-linux-gnuabi64
+
+# QEMU
+sudo apt-get install qemu-user-static
+sudo mkdir /etc/qemu-binfmt
+sudo ln -s /usr/mips64-linux-gnuabi64 /etc/qemu-binfmt/mips64
+```
+
+## CMake toolchain file
+
+Cross-compiling needs an correctly configured CMake, and the easiest way to
+do this consistently is to use a toolchain file. Create a `CMake-BE.toolchain`
+file with the following content in the root of the project:
+
+```
+# Operating system
+set(CMAKE_SYSTEM_NAME Linux)
+
+# Cross-compilers for C and C++
+set(CMAKE_C_COMPILER mips64-linux-gnuabi64-gcc)
+set(CMAKE_CXX_COMPILER mips64-linux-gnuabi64-g++)
+
+# Compiler environment
+set(CMAKE_FIND_ROOT_PATH /usr/mips64-linux-gnuabi64)
+
+# Default compiler and linker flags to use
+set(CMAKE_C_FLAGS_INIT -static)
+set(CMAKE_CXX_FLAGS_INIT -static)
+set(CMAKE_EXE_LINKER_FLAGS_INIT -static)
+set(CMAKE_SHARED_LINKER_FLAGS_INIT -static)
+set(CMAKE_MODULE_LINKER_FLAGS_INIT -static)
+
+# Never match host tools
+set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
+
+# Only match headers and libraries in the compiler environment
+set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
+set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
+```
+
+## Build astcenc
+
+Building uses CMake as normal, with the additional specification of the
+toolchain file to configure the build for cross-compilation.
+
+```
+cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=../ -DASTCENC_ISA_NONE=ON -DCMAKE_TOOLCHAIN_FILE=../CMake-BE.toolchain ..
+```
+
+## Run astcenc
+
+The cross-compiled `astcenc` binary runs as normal, and can access host files, but must run through QEMU to do the instruction-set translation.
+
+```
+qemu-mips64 ./bin/astcenc-none ...
+```
+
+- - -
+
+_Copyright © 2025, Arm Limited and contributors._