Build breaks on Rust 1.48

[hendrikhofstadt]

Problem

Solana does not build on Rust 1.48

error: attribute should be applied to a function
   --> core/src/validator.rs:953:9
    |
953 |           #[target_feature(enable = "avx")]
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
954 | /         {
955 | |             if is_x86_feature_detected!("avx") {
956 | |                 info!("AVX detected");
957 | |             } else {
...   |
960 | |             }
961 | |         }
    | |_________- not a function

This is likely due to changed checks for target_feature in the new Rust release:

https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1480-2020-11-19
rust-lang/rust#78143