Support for xtensa-esp8266-none-elf removed in 1.81

[nbaraz]

espup install no longer installs the xtensa-esp8266-none-elf target. It seems that the root cause is that the that the following commit (from the esp-1.80.0.0 branch):

commit b01f1613109b69a3b6ac2da4d47c79abe2422262
Author: Scott Mabin <scott@mabez.dev>
Date:   Sat Sep 12 23:36:21 2020 +0100

    Add some no_std Xtensa targets
    
    ESP32, ESP32S2, ESP32S3 & ESP8266.

...is not present in esp-1.81.0.0. A few other commits are not present as well.

[MabezDev]

I've added the target back in the 1.82 release. Please do note that we likely won't be upstreaming the 8266 target, so once the Xtensa LLVM upstreaming is complete you'll be stuck on the latest version of this fork.