Rollup merge of #149268 - davidtwco:v0-mangling-global-asm-namespace, r=Kivooeo

add implementation-internal namespace for globalasm

Fixes rust-lang/rust#138261

Adds a namespace for `global_asm` with a lowercase letter which [is reserved for implementation-internal disambiguation](https://doc.rust-lang.org/rustc/symbol-mangling/v0.html#namespace:~:text=Lowercase%20letters%20are%20reserved%20for%20implementation%2Dinternal%20disambiguation%20categories%20(and%20demanglers%20should%20never%20show%20them)):

> Lowercase letters are reserved for implementation-internal disambiguation categories (and demanglers should never show them)

As a implementation-internal disambiguation category, the demangler implementations shouldn't need updated (i.e. if this were an uppercase letter, then our mangle-then-demangle checks would fail because the demangler would expect to have explicit handling). `'a'` is chosen arbitrarily, for **a**sm, but I can change it to something else if preferred.

`#[rustc_symbol_name]` only looks at top-level items, and would need a bunch of changes to be able to check the symbol for `foo::{constant}::{closure}` in the `global_asm` in this test, so for now the test just checks this compiles.

The alternative to this would be to prohibit declaration of items in the operand of a `global_asm`, which is a breaking change.