[RFC] Don't use sys/types.h

openlibm includes `sys/types.h` in a few places and uses some non-standard types (e.g. `__uint32_t` and `u_int8_t`). This makes it hard to use in embedded environments (e.g. a Mirage unikernel). This patch allows openlibm to compile using the Mini-OS headers on x86_64 and arm32. I haven't tested other platforms. Is there any reason to use the full `sys/types.h`?