QEMU support for Xtensa

Things to do:

core/basic opcodes implementation;

[+] and/or/xor;

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

[+] call0, j

[+] l32r

MMU;

[+] no-mmu mode;

gdbserver;

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

[+] debug exception, single step mode;

[-] hw/sw breakpoints;

[-] wrong disassembly;

exceptions;

[+] debug;

sample evaluation board;

[+] memory mapping, ELF loader;

[-] standard periferals.