c-strtold: Add multithread-safety test. * tests/test-c-strtold-mt.c: New file. * tests/test-c-strtold-mt.sh: New file, based on tests/test-c-strtold1.sh. * modules/c-strtold-tests (Files): Add them. (Depends-on): Add thread, nanosleep. (Makefile.am): Arrange to build test-c-strtold-mt and run test-c-strtold-mt.sh.