This patch fixes the glibc testsuite build for GCC 7
-Wformat-truncation, newly moved out of -Wformat-length and with some
further warnings that didn't previously appear. Two tests that
previously disabled -Wformat-length are changed to disable
-Wformat-truncation instead; two others are made to disable that
option as well.
Tested (compilation only) with build-many-glibcs.py for aarch64 with
GCC mainline.
* stdio-common/tst-printf.c [__GNUC_PREREQ (7, 0)]: Ignore
-Wformat-truncation instead of -Wformat-length.
* time/tst-strptime2.c (mkbuf) [__GNUC_PREREQ (7, 0)]: Likewise.
* stdio-common/tstdiomisc.c (F): Ignore -Wformat-truncation for
GCC 7.
* wcsmbs/tst-wcstof.c: Include <libc-internal.h>.
(do_test): Ignore -Wformat-truncation for GCC 7.