Update riscv64 fenv.c initialization to fix warnings

[ViralBShah]

Mirrors the same change in aarch64.

[Copilot]

Pull Request Overview

This PR updates the default fenv_t environment initialization to use an aggregate initializer, addressing compiler warnings.

• Replace scalar 0 initializer with aggregate {0}.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.09%. Comparing base (9fbeafc) to head (2173bc0).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #330   +/-   ##
=======================================
  Coverage   72.09%   72.09%           
=======================================
  Files         233      233           
  Lines        6139     6139           
  Branches     1607     1607           
=======================================
  Hits         4426     4426           
  Misses       1420     1420           
  Partials      293      293           

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