lib/string_helpers.c: constify static arrays

lib/string_helpers.c: constify static arrays

Complement commit 68aecfb97978 ("lib/string_helpers.c: make arrays
static") by making the arrays const -- not only pointing to const
strings.  This moves them out of the data section to the r/o data
section:

   text    data     bss     dec     hex filename
   1150     176       0    1326     52e lib/string_helpers.old.o
   1326       0       0    1326     52e lib/string_helpers.new.o

Signed-off-by: Mathias Krause <[email protected]>
Cc: James Bottomley <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>