Cranelift/Wasmtime/Pulley/Debugging: use little-endian mode to spill/reload vectors in guest-debugging slot and ABI clobbers.

[cfallin]

When running Pulley on an s390x (or other big-endian) host, and enabling guest-debugging instrumentation, a very strange confluence of events occurs:

• Pulley uses "native endian" of the host by default for loads and stores.
• Patchable calls to debug hooks use the preserve_all ABI, which spills all registers in the trampoline adapter (callee in this ABI), including vector registers.
• Saving vector-typed locals/operand stack values to the debugger state slot also uses vector stores.
• All of these stores were thus big-endian on big-endian hosts.
• Pulley's bytecode only supports little-endian vector loads/stores.

We were thus hitting an assert in Pulley codegen (the Cranelift backend) when encountering a VStore VCode instruction with a big-endian mode.

This PR makes two changes that avoid this issue:

• The ABI code for Pulley is careful to specify little-endian mode explicitly for any vector load/store.
• The debug instrumentation code is refactored to use little-endian explicitly for vector types only.
  • (Why not for all types? Because we GC-root GC ref values, and these need to be provided to the collector as mutable storage cells, so need to be in native endianness.)

Test will come as part of #12575 incorporating a
Pulley-with-guest-debugging test and running on s390x amongst our platforms.