Fix widevine on aarch64

[HiassofT]

Description

The switch to hidden symbol visibility broke widevine on aarch64 as the functions in the aarch64 cdm_loader.cpp were no longer exported:

error <general>: AddOnLog: inputstream.adaptive: Initialize: Initialize: Failed to load library: /storage/.kodi/cdm/libwidevinecdm.so: undefined symbol: __aarch64_ldadd4_acq_rel

Add attributes to specify default visibility to those functions to make it work again.

Motivation and context

Make widevine work on aarch64 again

How has this been tested?

Playing ORF livestream with the ORF ON addon on RPi5 running LibreELEC 13

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

Checklist:

• I have read the Contributing document
• My code follows the Code Guidelines of this project
• My change requires a change to the Wiki documentation
• I have updated the documentation accordingly

[CastagnaIT]

thank you