lib: remove defining macros for strict_strto??

Open-code them rather than using defining macros.  The function bodies are now
next to their kerneldoc comments as a bonus.

Add casts to the signed cases as they call into the unsigned versions.

Avoids the sparse warnings:
lib/vsprintf.c:249:1: warning: incorrect type in argument 3 (different signedness)
lib/vsprintf.c:249:1:    expected unsigned long *res
lib/vsprintf.c:249:1:    got long *res
lib/vsprintf.c:249:1: warning: incorrect type in argument 3 (different signedness)
lib/vsprintf.c:249:1:    expected unsigned long *res
lib/vsprintf.c:249:1:    got long *res
lib/vsprintf.c:251:1: warning: incorrect type in argument 3 (different signedness)
lib/vsprintf.c:251:1:    expected unsigned long long *res
lib/vsprintf.c:251:1:    got long long *res
lib/vsprintf.c:251:1: warning: incorrect type in argument 3 (different signedness)
lib/vsprintf.c:251:1:    expected unsigned long long *res
lib/vsprintf.c:251:1:    got long long *res

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>