Update comments and sort target_arch in c_char_definition

[taiki-e]

Follow-up to #132975.

• Clang's wrong default on MSP430 has been fixed in [MSP430] Default to unsigned char llvm/llvm-project#115964, and will be included in LLVM 20, which will be used soon.
• Add a reference on Xtensa's default (from De-duplicate and improve definition of core::ffi::c_char #132975 (comment)).
• Fix link for Windows's default.
• Add a link to the discussion on L4Re (De-duplicate and improve definition of core::ffi::c_char #132975 (comment))
• Sort target_arch. (now match with target_archs in comments)

r? @tgross35