sd-boot: Fix PE section parsing

We only need the PE header offset from the DOS header, not
its size. Previously, the section table could be cut off in the middle.

While we are at it, also modernize the remaining code.

src/basic/macro.h

@@ -20,7 +20,6 @@
 #define _sentinel_ __attribute__((__sentinel__))
 #define _destructor_ __attribute__((__destructor__))
 #define _deprecated_ __attribute__((__deprecated__))
-#define _packed_ __attribute__((__packed__))
 #define _malloc_ __attribute__((__malloc__))
 #define _weak_ __attribute__((__weak__))
 #define _public_ __attribute__((__visibility__("default")))