I was able to get Ruby 2.5.1 building on M1 by following two of the steps in that GH issue...
 
https://github.com/rbenv/ruby-build/issues/1691#issuecomment-756272951
https://github.com/rbenv/ruby-build/issues/1691#issuecomment-772224551

I installed `readline` and `openssl` with standard `brew install`.

```bash
brew install readline
brew install openssl
```
 
Then I set this in my `~/.zshrc`..

```
# Homebrew
export PATH=/opt/homebrew/bin:$PATH
export PATH="/opt/homebrew/sbin:$PATH"
# rbenv
export RBENV_ROOT=/opt/homebrew/opt/rbenv
export PATH=$RBENV_ROOT/bin:$PATH
eval "$(rbenv init -)"
# openssl
export PATH="/opt/homebrew/opt/openssl@1.1/bin:$PATH"
export LDFLAGS="-L/opt/homebrew/opt/openssl@1.1/lib"
export CPPFLAGS="-I/opt/homebrew/opt/openssl@1.1/include"
export PKG_CONFIG_PATH="/opt/homebrew/opt/openssl@1.1/lib/pkgconfig"
export RUBY_CONFIGURE_OPTS="--with-openssl-dir=/opt/homebrew/opt/openssl@1.1"
```

Then I ran this command:

```
RUBY_CFLAGS="-Wno-error=implicit-function-declaration" rbenv install 2.5
```