gdbserver: Update require_int function to parse offset for pread packet

gdbserver: Update require_int function to parse offset for pread packet

Currently gdbserver uses the require_int() function to parse the
requested offset (in vFile::pread packet and the like).  This function
allows integers up to 0x7fffffff (to fit in 32-bit int), however the
offset (for the pread system call) has an off_t type which can be
larger than 32-bit.

This patch allows require_int() function to parse offset up to the
maximum value implied by the off_t type.

Approved-By: Pedro Alves <pedro@palves.net>
Change-Id: I3691bcc1ab1838c0db7f8b82d297d276a5419c8c