Deprecate 32 bit big-endian MIPS

It's becoming harder to maintain a cross-compiler to test this host
architecture as the old stable Debian 10 ("Buster") moved into LTS
which supports fewer architectures. For now:

  - mark it's deprecation in the docs
  - downgrade the containers to build TCG tests only
  - drop the cross builds from our CI

Users with an appropriate toolchain and user-space can still take
their chances building it.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Huacai Chen <chenhuacai@kernel.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220914155950.804707-22-alex.bennee@linaro.org>

docs/about/build-platforms.rst

@@ -41,7 +41,7 @@ Those hosts are officially supported, with various accelerators:
      - Accelerators
    * - Arm
      - kvm (64 bit only), tcg, xen
-   * - MIPS
+   * - MIPS (little endian only)
      - kvm, tcg
    * - PPC
      - kvm, tcg

docs/about/deprecated.rst

@@ -213,6 +213,19 @@ MIPS ``Trap-and-Emul`` KVM support (since 6.0)
 The MIPS ``Trap-and-Emul`` KVM host and guest support has been removed
 from Linux upstream kernel, declare it deprecated.
 
+Host Architectures
+------------------
+
+BE MIPS (since 7.2)
+'''''''''''''''''''
+
+As Debian 10 ("Buster") moved into LTS the big endian 32 bit version of
+MIPS moved out of support making it hard to maintain our
+cross-compilation CI tests of the architecture. As we no longer have
+CI coverage support may bitrot away before the deprecation process
+completes. The little endian variants of MIPS (both 32 and 64 bit) are
+still a supported host architecture.
+
 QEMU API (QAPI) events
 ----------------------