fix: cfg-gate the #![crate_name] and #![crate_type] attributes

[aatifsyed]

Description

I was vendoring libc as part of a C codebase that uses the meson build system.
This involved renaming the crate:

rust3p_libc = library('rust3p_libc', ...)

But these attributes clash with meson passing the --crate-name argument to rustc.

rustc -C linker=cc --color=always -C debug-assertions=yes -C overflow-checks=no --crate-type rlib -C opt-level=3 --crate-name rust3p_libc --emit dep-info=rust3p/rust3p_libc.d --emit link=rust3p/librust3p_libc.rlib --out-dir rust3p/librust3p_libc.rlib.p -C metadata=46de84a@@rust3p_libc@sta --edition=2015 --cfg 'feature="std"' ../rust3p/vendor/libc/src/lib.rs
error: `--crate-name` and `#[crate_name]` are required to match, but `rust3p_libc` != `libc`
 --> ../rust3p/vendor/libc/src/lib.rs:2:1
  |
2 | #![crate_name = "libc"]
  | ^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 1 previous error

It looks like this attribute was added to support "rustc-dep-of-std" in #28
Considering the rest of that code is #[cfg(feature = ...)] gated, moving this seems like the right the to do.

Sources

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

  ^ I can't run this on `main` locally :S
  

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