[Libc] add several specs

share/libc/signal.h

share/libc/stdlib.h

share/libc/string.h

share/libc/unistd.h

tests/builtins/oracle/imprecise.res.oracle

tests/builtins/oracle/memcpy.res.oracle

tests/builtins/oracle/memcpy2.res.oracle

tests/builtins/oracle/strnlen.res.oracle

tests/builtins/oracle/write-const.res.oracle

tests/libc/oracle/coverage.res.oracle

tests/libc/oracle/fc_libc.0.res.oracle

tests/libc/oracle/fc_libc.1.res.oracle

tests/libc/oracle/netdb_c.res.oracle

tests/libc/oracle/signal_h.res.oracle

tests/libc/oracle/stdlib_h.res.oracle

tests/libc/oracle/string_h.res.oracle

tests/libc/oracle/unistd_h.0.res.oracle

tests/libc/oracle/unistd_h.1.res.oracle

tests/libc/signal_h.c

tests/libc/stdlib_h.c