QEMU support for Xtensa

Things to do:

core/basic opcodes implementation;

[+] and/or/xor/neg/abs;

[+] shifts;

[+] add[x*]/sub[x*]/add.n/addi.n;

[+] call0, callx0, j, b*;

[+] l32*, s32*;

[+] windowed registers, call*/callx*, retw, rotw, rfwo, rfwu;

[-] boolean registers/commands;

windowed registers;

[+] simple overflow algorithm that's triggered from ENTER;

loop option;

floating point;

MMU;

[+] no-mmu mode;

gdbserver;

[+] xml register map, read/write register;

[+] debug exception, single step mode;

[-] hw/sw breakpoints;

[+] wrong disassembly (caused by wrong endianness of produced ELFs);

exceptions;

[+] debug;

[+] window overflow/underflow;

[-] invalid instruction;

sample evaluation board;

[+] memory mapping, ELF loader;

[-] standard periferals;

external configuration (a-la xtensa overlay)?