CSS order in Drupal 8 libraries.yml
You can create a separate library for external bootstrap stylesheet, which will give you flexibility to load bootstrap css before loading global-styling css. Here's how to do it:

YOURTHEMENAME.info.yml

# Defines libraries group in which we can add css/js.
libraries:
  - sp/bootstrap // ADDED - note: this should go before your global-styling
  - sp/global-styling
...


YOURTHEMENAME.libraries.yml

# note: bootstrap library should go before global-styling  
# use { type: external } for external resources
bootstrap:
      css:
        theme:
         '//maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css': { type: external }
global-styling:
      css:
    ...