ldbl-128ibm-compat: Compiler flags for stdio functions

Some of the files that provide stdio.h and wchar.h functions have a
filename prefixed with 'io', such as 'iovsprintf.c'.  On platforms that
imply ldbl-128ibm-compat, these files must be compiled with the flag
-mabi=ibmlongdouble.  This patch adds this flag to their compilation.

Notice that this is not required for the other files that provide
similar functions, because filenames that are not prefixed with 'io'
have ldbl-128ibm-compat counterparts in the Makefile, which already adds
-mabi=ibmlongdouble to them.

Reviewed-by: Gabriel F. T. Gomes <gabrielftg@linux.ibm.com>