Update rust-bpf-builder to be based on Rust 1.40 or above

[Aaron1011]

Currently, rust-bpf-builder uses a custom version of Rust based on Rust 1.39: https://github.com/solana-labs/rust-bpf-builder/blob/2ae3f921f65789067963b7d7a41219d0a1997edb/linux/Dockerfile#L66-L69

In Rust 1.40, procedural macros can expand to macro_rules! declaration without using #![feature(proc_macro_hygiene)]. By basing rust-bpf-builder on a newer version of Rust, we can avoid the need to conditionally apply the #![feature(proc_macro_hygiene)] feature gate in #10905

[garious]

@jackcmay ^^^

[jackcmay]

@garious Yup

[Lichtso]

I rebased the BPF patches onto Rust 1.46
anza-xyz/rust#2
https://github.com/solana-labs/rust/tree/solana-1.46
https://github.com/solana-labs/llvm-project/tree/solana-rustc/10.0-2020-05-05

It still compiles on macOS, but I haven't done any further testing yet.

[jackcmay]

Rust v1.46 is planned for Solana v1.6 and support for v1.50 and incremental upgrades after is in the works