Make strtoimax, strtoumax, wcstoimax, wcstoumax into aliases

The functions strtoimax, strtoumax, wcstoimax, wcstoumax currently
have three implementations each (wordsize-32, wordsize-64 and dummy
implementation in stdlib/ using #error), defining the functions as
thin wrappers round corresponding *_internal functions.  Simplify the
code by changing them into aliases of functions such as strtol and
wcstoull.  This is more consistent with how e.g. imaxdiv is handled.

Tested for x86_64 and x86.