fix incorrect results in expm1(::Union{Float16, Float32}) (#50989)

`unsafe_trunc(UInt, -1.0)` is unspecified behavior but worked fine on
apple and AMD so we didn't notice??? This has been very broken since 1.7.

(cherry picked from commit 61ebaf6)

Author: oscardssmith

base/special/exp.jl

@@ -460,7 +460,7 @@ function expm1(x::Float32)
     end
     x = Float64(x)
     N_float = round(x*Ln2INV(Float64))
-    N = unsafe_trunc(UInt64, N_float)
+    N = unsafe_trunc(Int64, N_float)
     r = muladd(N_float, Ln2(Float64), x)
     hi = evalpoly(r, (1.0, .5, 0.16666667546642386, 0.041666183019487026,
                       0.008332997481506921, 0.0013966479175977883, 0.0002004037059220124))
@@ -477,7 +477,7 @@ function expm1(x::Float16)
         return Float16(x*evalpoly(x, (1f0, .5f0, 0.16666628f0, 0.04166785f0, 0.008351848f0, 0.0013675707f0)))
     end
     N_float = round(x*Ln2INV(Float32))
-    N = unsafe_trunc(UInt32, N_float)
+    N = unsafe_trunc(Int32, N_float)
     r = muladd(N_float, Ln2(Float32), x)
     hi = evalpoly(r, (1f0, .5f0, 0.16666667f0, 0.041665863f0, 0.008333111f0, 0.0013981499f0, 0.00019983904f0))
     small_part = r*hi

test/math.jl

@@ -188,6 +188,7 @@ end
             @test exp10(x) ≈ exp10(big(x))
             @test exp2(x) ≈ exp2(big(x))
             @test expm1(x) ≈ expm1(big(x))
+            @test expm1(T(-1.1)) ≈ expm1(big(T(-1.1)))
             @test hypot(x,y) ≈ hypot(big(x),big(y))
             @test hypot(x,x,y) ≈ hypot(hypot(big(x),big(x)),big(y))
             @test hypot(x,x,y,y) ≈ hypot(hypot(big(x),big(x)),hypot(big(y),big(y)))