Rename and better describe spill/reload from stack in oemit_flow.c

  The previous x_spill and x_load macros were renamed and a comment
added about its use, that is to not spill/reload from the language
thread state, but directly from the stack.
Silence warnings about _BSD_SOURCE.

  In recent glibc it is an alias to predefined _DEFAULT_SOURCE, and a
warning about it is issued for every file being compiled.
Correct build on systems without tls.

  This was done in a build forcing it to not recognize tls. Note that
pthread is mandatory, but tls is optional, to support a few extra
systems.
Add simple read and write builtin functions.

  These functions allow reading and writing binary data to/from streams
and can be useful for some type casts/conversions or any operation that
requires raw float/integer read/write.
Only base types are allowed as it cannot allow reading raw pointers from
streams (for consistency write is also an error).