riscv: Fix feholdexcept()

[AlekseyZhmulin]

The feholdexcept() function must store the current floating point environment in *__envp. Related to #321

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 72.09%. Comparing base (c9c6fd6) to head (361868e).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #324   +/-   ##
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  Coverage   72.09%   72.09%           
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  Files         233      233           
  Lines        6139     6139           
  Branches     1607     1607           
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  Hits         4426     4426           
  Misses       1420     1420           
  Partials      293      293           

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[AlekseyZhmulin]

The problem is that feholdexcept() does not save floating point environment, but feupdateenv() loads floating point environment. Therefore, in the lrint() function, "fenv_t env" is not initialized and a random value from the stack is loaded into the fcsr register. https://github.com/JuliaMath/openlibm/blob/master/src/s_lrint.c#L59. This throws an "Illegal instruction" exception.

[ViralBShah]

Musl: https://git.musl-libc.org/cgit/musl/tree/src/fenv/feholdexcept.c?id=6f2e5607d2d463dfc342489660e089553b749dcf

[ViralBShah]

@inkydragon Thoughts on whether we should merge?

[AlekseyZhmulin]

I also added clearing of all exception flags in feholdexcept()