compiler-rt: compute correct integer sizes from bits at runtime

Also, accepting `align(1)` pointers ensures that the alignment is safety
checked rather than assumed.

lib/compiler_rt/fixdfei.zig

@@ -1,6 +1,7 @@
-const divCeil = @import("std").math.divCeil;
-const common = @import("./common.zig");
-const bigIntFromFloat = @import("./int_from_float.zig").bigIntFromFloat;
+const std = @import("std");
+const builtin = @import("builtin");
+const common = @import("common.zig");
+const bigIntFromFloat = @import("int_from_float.zig").bigIntFromFloat;
 
 pub const panic = common.panic;
 
@@ -8,6 +9,7 @@ comptime {
     @export(&__fixdfei, .{ .name = "__fixdfei", .linkage = common.linkage, .visibility = common.visibility });
 }
 
-pub fn __fixdfei(r: [*]u32, bits: usize, a: f64) callconv(.c) void {
-    return bigIntFromFloat(.signed, r[0 .. divCeil(usize, bits, 32) catch unreachable], a);
+pub fn __fixdfei(r: [*]u8, bits: usize, a: f64) callconv(.c) void {
+    const byte_size = std.zig.target.intByteSize(builtin.target, @intCast(bits));
+    return bigIntFromFloat(.signed, @ptrCast(@alignCast(r[0..byte_size])), a);
 }