Rollup merge of #40702 - mrhota:global_asm, r=nagisa

Implement global_asm!() (RFC 1548)

This is a first attempt. ~~One (potential) problem I haven't solved is how to handle multiple usages of `global_asm!` in a module/crate. It looks like `LLVMSetModuleInlineAsm` overwrites module asm, and `LLVMAppendModuleInlineAsm` is not provided in LLVM C headers 😦~~

I can provide more detail as needed, but honestly, there's not a lot going on here.

r? @eddyb

CC @Amanieu @jackpot51

Tracking issue: #35119

diff --cc src/doc/unstable-book/src/SUMMARY.md
Simple merge

diff --cc src/librustc_metadata/encoder.rs
Simple merge