rust: run libc build script (#4336)

Summary:
When setting up the transitive dep on `libc` in #4316, I had originally
passed `--cfg=libc_align`, because that fixed the build for me and on
CI, and I couldn’t figure out the right cross-platform fix. But @faern
[helpfully points out that we just need to run the build script][1],
which detects the OS and Rust compiler version to determine the
appropriate flags. Bazel `rules_rust` can handle this, if we just ask.

(I should have followed my own `DEVELOPMENT.md` suggestions!)

[1]: rust-lang/socket2#130 (comment)

Test Plan:
It still builds—`bazel build @raze__libc__0_2_80//:libc`—and the demo
server still works and accepts requests following #4318’s test plan.

wchargin-branch: rust-libc-gen-buildrs

tensorboard/data/server/Cargo.toml

@@ -60,9 +60,7 @@ targets = [
 gen_buildrs = true
 
 [package.metadata.raze.crates.libc.'0.2.80']
-additional_flags = [
-    "--cfg=libc_align",
-]
+gen_buildrs = true
 
 [package.metadata.raze.crates.indexmap.'1.6.0']
 additional_flags = [