Use a local buffer in modf

Fix #21591

base/math.jl

@@ -695,16 +695,16 @@ julia> modf(3.5)
 """
 modf(x) = rem(x,one(x)), trunc(x)
 
-const _modff_temp = Ref{Float32}()
 function modf(x::Float32)
-    f = ccall((:modff,libm), Float32, (Float32,Ptr{Float32}), x, _modff_temp)
-    f, _modff_temp[]
+    temp = Ref{Float32}()
+    f = ccall((:modff, libm), Float32, (Float32, Ptr{Float32}), x, temp)
+    f, temp[]
 end
 
-const _modf_temp = Ref{Float64}()
 function modf(x::Float64)
-    f = ccall((:modf,libm), Float64, (Float64,Ptr{Float64}), x, _modf_temp)
-    f, _modf_temp[]
+    temp = Ref{Float64}()
+    f = ccall((:modf, libm), Float64, (Float64, Ptr{Float64}), x, temp)
+    f, temp[]
 end
 
 @inline ^(x::Float64, y::Float64) = nan_dom_err(ccall("llvm.pow.f64", llvmcall, Float64, (Float64, Float64), x, y), x + y)