[llvm] [Bazel] Set HAVE_MALLINFO2=1 (PR #125681)

[Jordan Rupprecht via llvm-commits]

rupprecht wrote:

`mallinfo2` is relatively new, so while it should be available in a lot of places, it won't work on all versions of glibc. I don't know exact years/versions, but I think it's recent enough that we need to care about it. It'd be nice if we could autodetect this from bazel.

The bazelrc file in llvm sets `-Wno-deprecated` for both `--config=generic_gcc` and `--config=generic_clang`. Maybe we could add `copts = ["-Wno-deprecated-declarations"]` to this build target?

https://github.com/llvm/llvm-project/pull/125681