[clang] [Xtensa] Default to unsigned char (PR #115967)

[Alexey Gerenkov via cfe-commits]

gerekon wrote:

> Thanks, do you believe we need a backwards compatibility flag for the ABI to be consistent with older clang or is this safe to land?

I think as far as we can control behavior with `-fsigned-char` we do not need another option for this. 
@MabezDev WDYT? Will it be critical for Rust builds?

https://github.com/llvm/llvm-project/pull/115967