Fix recent bugs with shim/third-party

Summary:
Observed builds at [TraceMachina/buck2](https://github.com/TraceMachina/buck2/actions/workflows/build-and-test.yml) recently broke due to inconsistencies with shim/third-party directory.

`reindeer buckify` will not delete existing `Cargo.lock` and `BUCK` if they already exist. `reindeer update` might assume that `reindeer buckify` was done first. A simple and consistent way is to ensure those files are deleted before calling `reindeer` instead of assuming how `reindeer` will behave.

[serde-rs](https://github.com/serde-rs/json/blob/5ebf65cc480f90714c94f82099ca9161d80cbb10/build.rs#L11) recently introduced a rustc configuration flag that has to be set.`fast_arithmetic` chooses the architecture based on `target_arch`, repeat the same in the fixup scripts.

`zstd` is missing header files, most likely not caught due to stale `Cargo.lock` and `BUCK` files.

X-link: facebook/buck2#751

Reviewed By: JakobDegen

Differential Revision: D61753931

Pulled By: dtolnay

fbshipit-source-id: 2432083b700149a24cdd9b3954d250a147403954

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

@@ -1 +1,2 @@
-buildscript = []
+[[buildscript]]
+[buildscript.rustc_flags]

shim/third-party/rust/fixups/zstd-sys/fixups.toml

@@ -21,6 +21,8 @@ srcs = [
 headers = [
  "zdict.h",
  "zstd.h",
+ "zstd/lib/common/allocations.h",
+ "zstd/lib/common/bits.h",
  "zstd/lib/common/bitstream.h",
  "zstd/lib/common/compiler.h",
  "zstd/lib/common/cpu.h",
@@ -32,7 +34,7 @@ headers = [
  "zstd/lib/common/pool.h",
  "zstd/lib/common/portability_macros.h",
  "zstd/lib/common/threading.h",
- #"zstd/lib/common/xxhash.h",
+ "zstd/lib/common/xxhash.h",
  "zstd/lib/common/zstd_deps.h",
  "zstd/lib/common/zstd_internal.h",
  "zstd/lib/common/zstd_trace.h",
@@ -78,6 +80,8 @@ srcs = [
 headers = [
  "zdict.h",
  "zstd.h",
+ "zstd/lib/common/allocations.h",
+ "zstd/lib/common/bits.h",
  "zstd/lib/common/bitstream.h",
  "zstd/lib/common/compiler.h",
  "zstd/lib/common/cpu.h",
@@ -89,7 +93,7 @@ headers = [
  "zstd/lib/common/pool.h",
  "zstd/lib/common/portability_macros.h",
  "zstd/lib/common/threading.h",
- #"zstd/lib/common/xxhash.h",
+ "zstd/lib/common/xxhash.h",
  "zstd/lib/common/zstd_deps.h",
  "zstd/lib/common/zstd_internal.h",
  "zstd/lib/common/zstd_trace.h",