Rollup merge of #61720 - alexcrichton:libstd-cfg-if-dep, r=sfackler

std: Remove internal definitions of `cfg_if!` macro

This is duplicated in a few locations throughout the sysroot to work
around issues with not exporting a macro in libstd but still wanting it
available to sysroot crates to define blocks. Nowadays though we can
simply depend on the `cfg-if` crate on crates.io, allowing us to use it
from there!

diff --cc Cargo.lock
Simple merge

diff --cc src/libstd/macros.rs
Simple merge