Implement fcvt_to_uint_sat (f32x4 -> i32x4) for x86

[abrown]

This replaces #1822; it consists of the same functionality but removes the AVX512 instruction lowering for the time being. There are two reasons for this:

• the default MXCSR rounding is round to nearest even, which does not match the semantics required by i32x4.trunc_sat_f32x4_u. We can then use embedded rounding control but lose the ability to specify the vector length, so the instruction would operate on 512-bits which we should discuss (@sunfishcode has reported issues with 512-bit vectors in Spidermonkey)
• the output of VCVTPS2UDQ for negative lanes is 0xFFFFFFFF (I had thought it would be 0x00000000); this can be resolved with the following sequence: v0 = pxor ...; v2 = fcmp gte v1, v0 (gte ensures they are ordered); v3 = vcvtps2udq v1; v4 = band v2, v3. However, I would like to look at this a little bit more before submitting a separate PR for it (this is the reason for keeping the legalization in enc_tables.rs and under narrow_avx, BTW).

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[julian-seward1]

Ok to land, but please remove the redundant mention of AVX512 in the commit message:
"This converts an f32x4 into an i32x4 (unsigned) with some rounding either by using an AVX512VL/F instruction--VCVTPS2UDQ--or a long sequence of SSE4.1 compatible instructions."

Thanks for your patience with this!