Add a basic ldexp

Enough to make ziglang#23358 snipped pass

Author: James Cox-Morton

lib/compiler_rt.zig

@@ -219,6 +219,7 @@ comptime {
     _ = @import("compiler_rt/fmax.zig");
     _ = @import("compiler_rt/fmin.zig");
     _ = @import("compiler_rt/fmod.zig");
+    _ = @import("compiler_rt/ldexp.zig");
     _ = @import("compiler_rt/log.zig");
     _ = @import("compiler_rt/log10.zig");
     _ = @import("compiler_rt/log2.zig");

lib/compiler_rt/ldexp.zig

@@ -0,0 +1,18 @@
+const std = @import("std");
+const expect = std.testing.expect;
+const math = std.math;
+const common = @import("common.zig");
+
+comptime {
+    @export(&ldexp, .{ .name = "ldexp", .linkage = common.linkage, .visibility = common.visibility });
+}
+
+pub fn ldexp(x: f64, n: i32) callconv(.c) f64 {
+    return math.scalbn(x, n);
+}
+
+test "ldexp_64" {
+    // Ported from libc-test
+    // https://repo.or.cz/libc-test.git/blob/HEAD:/src/math/sanity/ldexp.h
+    try expect(ldexp( -0x1.02239f3c6a8f1p+3, -2) == -0x1.02239f3c6a8f1p+1);
+}