wasmtime: Enable SIMD testsuite for RISC-V

[afonso360]

👋 Hey,

This PR enables some of the Wasmtime SIMD testsuite for RISC-V. Currently we pass the following tests:

wast::Cranelift::misc::simd::issue_3173_select_v128
wast::Cranelift::misc::simd::unaligned_load
wast::Cranelift::spec::simd_address
wast::Cranelift::spec::simd_i16x8_arith
wast::Cranelift::spec::simd_linking
wast::Cranelift::spec::simd_i64x2_arith
wast::Cranelift::spec::simd_i8x16_arith
wast::Cranelift::spec::simd_i32x4_arith
wast::Cranelift::spec::simd_store
wast::Cranelift::spec::simd_const

[jameysharp]

Exciting!