Add statvfs and strtold

build.rs

include/bits/stdlib.h

include/sys/types.h

src/c/stdlib.c

src/header/mod.rs

src/header/stdlib/cbindgen.toml

src/header/sys_statvfs/cbindgen.toml

src/header/sys_statvfs/mod.rs

src/platform/linux/mod.rs

src/platform/pal/mod.rs

src/platform/redox/mod.rs

src/platform/types.rs