selftests/nolibc: scope custom flags to the nolibc-test target
A new target for 'libc-test' is going to be added which should not be
affected by these options.
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://patch.msgid.link/20260106-nolibc-selftests-v1-2-f82101c2c505@weissschuh.net