Switch to force loading std for panic_handler

Summary:
When building buck sysroot from source, `#[no_std]` crates would fail to link as shared libraries, citing "missing #[panic_handler]".

Matt introduced a mechanism in rust-lang/rust#109421 that allows us to still link in the default panic handler from `std`, even if the rest of the crate is completely unused (as is the case with `no_std` crates).

The net result is that we no longer need to force static linkage when building no_std crates with buckified sysroot.

Reviewed By: zertosh

Differential Revision: D47402849

fbshipit-source-id: 35e31c7ff6d5683ff32a3c1eb2d67acec57acc8f

Author: capickett

shim/third-party/rust/fixups/inventory/fixups.toml

@@ -1,3 +1 @@
-# #[no_std] cannot be built with shared linkage
-preferred_linkage = "static"
 buildscript = []