... instead, just calculate the value unless it is too big.
Also, this change raises an ArgumentError if it is expected to exceed
16 GB in a 64-bit environment.

(It is possible to calculate it straightforward, but it would likely be
out-of-memory, so I didn't think it would make sense.)

[Feature #20811]

The spec is actually testing a behaviour stemming from NUM2INT(), and
since `sizeof(long)>=sizeof(int)`, `min_long-1` always makes NUM2INT()
raise `RangeError`.

What a "word" is when talking about sizes is confusing because it's a
highly overloaded term. Intel, Microsoft, and GDB are just a few vendors
that have their own definition of what a "word" is. Specs that used the
"wordsize" guard actually were mostly testing for the size of the C
`long` fundamental type, so rename the guard for clarity.

Also, get the size of `long` directly from RbConfig instead of guessing
using Integer#size. Integer#size is not guaranteed to have anything to
do with the `long` type.