warning on riscv64

[inkydragon]

Moved from https://github.com/JuliaMath/openlibm/issues/278#issuecomment-2581246607

There's the same warning on riscv64:

In file included from /home/runner/work/openlibm/openlibm/include/openlibm_fenv.h:18,
                 from src/s_lrint.c:29:
In function ‘feupdateenv’,
    inlined from ‘lrint’ at src/s_lrint.c:59:2:
/home/runner/work/openlibm/openlibm/include/openlibm_fenv_riscv.h:87:25: warning: ‘env’ is used uninitialized [-Wuninitialized]
   87 | #define __wfs(__fcsr)   __asm __volatile("csrw fcsr, %0" :: "r" (__fcsr))
      |                         ^~~~~
/home/runner/work/openlibm/openlibm/include/openlibm_fenv_riscv.h:213:9: note: in expansion of macro ‘__wfs’
  213 |         __wfs(*__envp);
      |         ^~~~~
src/s_lrint.c: In function ‘lrint’:
src/s_lrint.c:52:16: note: ‘env’ was declared here
   52 |         fenv_t env;
      |                ^~~

This also makes Julia crash on riscv64 during bootstrap after the update of openlibm to v0.8.4 with the following error message

$ make -j3
[...]
    JULIA usr/lib/julia/sys-o.a
Collecting and executing precompile statements
└ Collect (Basic: ✓ fatal: error thrown and no exception handler available.
BoundsError(a=Array{Base.Partr.taskheap, 1}(dims=(0,), mem=Memory{Base.Partr.taskheap}(0, 0x3f848c0a60)[]), i=(0,))
throw_boundserror at ./essentials.jl:15
getindex at ./essentials.jl:916 [inlined]
getindex at ./abstractarray.jl:1345 [inlined]
multiq_insert at ./partr.jl:152
enq_work at ./task.jl:972
#schedule#589 at ./task.jl:1040
schedule at ./task.jl:1027 [inlined]
notify at ./condition.jl:165
#notify#552 at ./condition.jl:159 [inlined]
notify at ./condition.jl:159 [inlined]
notify at ./condition.jl:159 [inlined]
task_done_hook at ./task.jl:823
unknown function (ip: 0x3f7e650957) at (unknown file)
jl_apply at /home/mose/repo/julia/src/julia.h:2246 [inlined]
jl_finish_task at /home/mose/repo/julia/src/task.c:345
start_task at /home/mose/repo/julia/src/task.c:1292
*** This error is usually fixed by running `make clean`. If the error persists, try `make cleanall`. ***
make[1]: *** [sysimage.mk:99: /home/mose/repo/julia/usr/lib/julia/sys-o.a] Error 1
make: *** [Makefile:120: julia-sysimg-release] Error 2
Command exited with non-zero status 2

On that commit, Julia uses fgetenv/fsetenv inside the task scheduler, which matches the stacktrace. However the use of fenv was removed a few days afterwards, but this still looks to me like a strong indication that the fenv code on riscv64 is bad.

CC: @xctan who introduced this code in #254