c-strtod: Add multithread-safety test. * tests/test-c-strtod-mt.c: New file. * tests/test-c-strtod-mt.sh: New file, based on tests/test-c-strtod1.sh. * modules/c-strtod-tests (Files): Add them. (Depends-on): Add thread, nanosleep. (Makefile.am): Arrange to build test-c-strtod-mt and run test-c-strtod-mt.sh.