Add abi.export_name.null

I'm guessing this is a reasonable rule to have for all targets. At
least rustc treats it that way.
https://github.com/rust-lang/rust/blob/5d707b07e42766c080c5012869c9988a18dcbb83/compiler/rustc_codegen_ssa/src/codegen_attrs.rs#L254-L264

Author: ehuss

src/abi.md

@@ -173,6 +173,9 @@ If `export_name` is used with [`no_mangle`][abi.no_mangle], then the `export_nam
 r[abi.export_name.publicly-exported]
 The `export_name` attribute causes the symbol to be publicly exported from the produced library or object file, similar to the [`used` attribute](#the-used-attribute).
 
+r[abi.export_name.null]
+The exported name must not contain a [NUL] character.
+
 [attribute]: attributes.md
 [extern functions]: items/functions.md#extern-function-qualifier
 [external blocks]: items/external-blocks.md