pulley: Ungate SIMD proposal to enable more tests

[alexcrichton]

This commit removes the simd proposal from "known panicking compiler
features" for Pulley by filling out some minor ABI bits around
loads/stores of v128 in the Cranelift ABI code. This unlocks running
spec_testsuite/*.wast in CI and expecting it to pass as otherwise
everything was rejected since that test suite requires SIMD and it never
even got to the Cranelift backend.

This then fixes a few minor issues here and there to get existing tests
passing now that they're actually being run. Some examples are:

• Fix some copy/paste typos around callee-save clobbers and float
  registers.
• Add vector load/store support to avoid panicking in ABI code (not well
  tested yet though).
• Zero-extend the condition in brif's lowering to ensure that the
  entire register is defined. In the future more variants of br_if in
  Pulley are probably desired.
• Limit store lowerings to just integer types, not vector types.
• Fix Func::call with Pulley when the callee is a host function.

cc #9783

[alexcrichton]

Note that this is currently based on #9775

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[fitzgen]

Nice