Fix std.c.EXC.MASK struct to match definition in mach/exception_types.h

[danielchasehooper]

This change fixes two issues:

1. The Zig definition of std.c.EXC.MASK had the mask flags starting at bit 0, but the header it's copying has this:

#define EXC_BAD_ACCESS          1

...

/*
 * Masks for exception definitions, above
 * bit zero is unused, therefore 1 word = 31 exception types
 */

#define EXC_MASK_BAD_ACCESS             (1 << EXC_BAD_ACCESS)

2. the structure wasn't padded to 32 bits

Obviously the mask isn't used anywhere is zig, because it would've caused a packed structure padding compile error. On the chance that it is used in the future, either by the compiler, or as an api provided by std, I figured I'd save future poor souls from mysterious bugs caused by the mask flags bits being off by one bit.

[alexrp]

Just some naming nitpicks; for unused/padding fields in packed structs that mirror C enums, we prefer to use the bit position as the name.

Otherwise LGTM.