doc: arch: semihost: Add Xtensa support

Overview

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Semihosting is a mechanism that enables code running on ARM and RISC-V targets

to communicate and use the Input/Output facilities on a host computer that is

running a debugger or emulator.

Semihosting is a mechanism that enables code running on ARM, RISC-V and Xtensa

targets to communicate and use the Input/Output facilities on a host computer

that is running a debugger or emulator.

More complete documentation on the available functionality is available at the

`ARM Github documentation`_.

The RISC-V functionality borrows from the ARM definitions, as described at the

`RISC-V Github documentation`_.

Semihosting implementation on Xtensa supports GDB File-I/O extension, which is

explained in `GDB File-I/O Remote Protocol`_.

File Operations

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.. _ARM Github documentation: https://github.com/ARM-software/abi-aa/blob/main/semihosting/semihosting.rst

.. _RISC-V Github documentation: https://github.com/riscv/riscv-semihosting-spec/blob/main/riscv-semihosting-spec.adoc

.. _GDB File-I/O Remote Protocol: https://sourceware.org/gdb/current/onlinedocs/gdb.html/File_002dI_002fO-Remote-Protocol-Extension.html