Fix long double on x86_64-windows

The larger alignment on this platform means that long double reports
a sizeof 16 bytes, but it's underlying size is really just the 10
bytes of `f80`

C doesn't give us a way to see the "underlying" size of a type, so
this has to be caught by hand or by monitoring runtime memory. Luckily,
x86 and x86-64 are the only platforms that seem to use a non-power-of-two
type like this.

src/type.zig

@@ -6827,7 +6827,7 @@ pub const CType = enum {
                     },
                     .longlong, .ulonglong, .double => return 64,
                     .longdouble => switch (target.abi) {
-                        .gnu, .gnuilp32, .cygnus => return 128,
+                        .gnu, .gnuilp32, .cygnus => return 80,
                         else => return 64,
                     },
                 },