Add supported cross compile targets to README

[zmichaels11]

Description

Supported target platforms should be easily located in the README doc so users of Cross Compile can quickly know what is supported.

Test Plan

• N/A

Documentation

• Update README.md to display supported cross compile targets

Release Plan

• check in PR

Acceptance Criteria

• [ ] README.md is updated
• [ ] Release plan has been completed

Once all above items are checked, this story can be moved to done.

Resources

[prajakta-gokhale]

This will be covered in https://github.com/ros-tooling/cross_compile/issues/69.

[emersonknapp]

To support this, I am making minor changes to the sysroot_compiler to centralize supported targets in one place (in addition to the README update)