arm: support FEAT_AFP FTZ/DAZ semantics and tests

[howjmay]

Map FTZ and DAZ independently on ARMv8.7+ FEAT_AFP by using FPCR.FZ and FPCR.FIZ with AH handling, while preserving the pre-AFP unified fallback. Update _mm_setcsr/_mm_getcsr and denormal helpers accordingly, and expand IEEE754 tests to validate AFP independence plus legacy ARM coupling behavior.

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Summary by cubic

Add independent FTZ (outputs) and DAZ (inputs) on ARMv8.7+ with FEAT_AFP by mapping FTZ to FPCR.FZ and DAZ to FPCR.FIZ, with FPCR.AH set when either is enabled. Keeps the unified FTZ+DAZ fallback on older ARM; updates MXCSR helpers and tests to verify AFP independence and legacy coupling.

• New Features
  • FEAT_AFP (ARMv8.7+): FTZ → FPCR.FZ, DAZ → FPCR.FIZ; set FPCR.AH when FZ or FIZ is on.
  • Pre-AFP fallback: DAZ unified with FTZ via FPCR.FZ.
  • _mm_setcsr/_mm_getcsr: read/write FZ and FIZ independently on AFP; preserve unified behavior otherwise.
  • Denormal helpers (_sse2neon_mm_get_denormals_zero_mode/_sse2neon_mm_set_denormals_zero_mode): use FIZ on AFP.
  • IEEE754 tests: update round-trip expectations and add AFP-only tests for FTZ-only (no input flush) and DAZ-only (no output flush).

^{Written for commit eea7145. Summary will update on new commits.}

[jserv]

update?