arch: xtensa: Add semihosting support

Add semihosting support for Xtensa architecture.

Existing semihosting instructions are based on ARM, so they are
converted to Xtensa codes before the semihosting call is invoked.

Return codes of read, write and seek calls had to be converted to match
semihosting API definitions.

Signed-off-by: Tahsin Mutlugun <Tahsin.Mutlugun@analog.com>