Make INV_2PI a tuple

Without this, the compiler cannot assume that the range reduction
is idempotent to make use of the new fast constprop code path.
In the future this could potentially be an ImmutableArray, but
since this is relatively small, a tuple is probably fine.

base/special/rem_pio2.jl

@@ -23,7 +23,7 @@
 #        @printf "0x%016x,\n" k
 #        I -= k
 #    end
-const INV_2PI = UInt64[
+const INV_2PI = (
     0x28be_60db_9391_054a,
     0x7f09_d5f4_7d4d_3770,
     0x36d8_a566_4f10_e410,
@@ -42,7 +42,7 @@ const INV_2PI = UInt64[
     0x5d49_eeb1_faf9_7c5e,
     0xcf41_ce7d_e294_a4ba,
     0x9afe_d7ec_47e3_5742,
-    0x1580_cc11_bf1e_daea]
+    0x1580_cc11_bf1e_daea)
 
 @inline function cody_waite_2c_pio2(x::Float64, fn, n)
     pio2_1 = 1.57079632673412561417e+00