Write entire buffer in gdbserver write_prim

Write entire buffer in gdbserver write_prim

We had a customer bug report which was eventually tracked down to
gdbserver not fully sending a target description to gdb.  (This
presented as a timeout on the gdb side.)

The customer was using the WINAPI code, which does this:

  # define write(fd, buf, len) send (fd, (char *) buf, len, 0)

In this setup, I think it's possible to have a partial write.
However, gdbserver does not account for this possibility, despite the
fact that write_prim documents this.

This patch attempts to fix the problem by always writing the full
buffer in write_prim.  In this case the customer fixed their bug in a
different way, so we haven't actually tested this in the wild.

v2: Return bool from write_prim.

Reviewed-by: Kévin Le Gouguec <legouguec@adacore.com>