Rollup merge of #66235 - eddyb:coff-syrup, r=nagisa

rustc_metadata: don't let LLVM confuse rmeta blobs for COFF object files.

This has likely been a silent issue since 1.10 but only caused trouble recently (see https://github.com/rust-lang/rust/issues/65536#issuecomment-552018224), when recent changes to the `rmeta` schema introduced more opportunities for COFF parse errors.

To prevent any undesired interactions with old compilers, I've renamed the file inside `rlib`s from `rust.metadata.bin` to `lib.rmeta` (not strongly attached to it, suggestions welcome).

Fixes #65536.

<hr/>

Before:
```
$ llvm-objdump -all-headers build/*/stage1-std/*/release/deps/libcore-*.rmeta

build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/release/deps/libcore-6b9e8b5a59b79a1d.rmeta: file format COFF-<unknown arch>

architecture: unknown
start address: 0x00000000

Sections:
Idx Name          Size     VMA          Type

SYMBOL TABLE:
```

After:
```
$ llvm-objdump -all-headers build/*/stage1-std/*/release/deps/libcore-*.rmeta

llvm-objdump: error: 'build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/release/deps/libcore-6b9e8b5a59b79a1d.rmeta':
    The file was not recognized as a valid object file
```

1 files changed, 2 insertions, 1 deletions

diff --git a/src/librustc_codegen_ssa/lib.rs b/src/librustc_codegen_ssa/lib.rs
index dd75883f97d..81e7ef64e97 100644
--- a/src/librustc_codegen_ssa/lib.rs
+++ b/src/librustc_codegen_ssa/lib.rs
@@ -59,7 +59,8 @@ pub struct ModuleCodegen<M> {
     pub kind: ModuleKind,
 }
 
-pub const METADATA_FILENAME: &str = "rust.metadata.bin";
+// FIXME(eddyb) maybe include the crate name in this?
+pub const METADATA_FILENAME: &str = "lib.rmeta";
 pub const RLIB_BYTECODE_EXTENSION: &str = "bc.z";