ecs-gpu-init: clean up linker flags

[yeazelm]

This commit was cherry picked from #3887 since we took the SDK where the change was needed. This avoids building ecs-gpu-init with broken symbols which show up with these errors in the system log:

[   14.946273] ecs-gpu-init[1868]: /usr/bin/ecs-gpu-init: symbol lookup error: /usr/bin/ecs-gpu-init: undefined symbol: nvmlGpuInstanceGetComputeInstanceProfileInfoV
[FAILED] Failed to start Initialize ECS GPU config.
See 'systemctl status ecs-gpu-init.service' for details.
[DEPEND] Dependency failed for Bottlerocket initial configuration complete.
[DEPEND] Dependency failed for Isolates configured.target.

From the original commit:
--export-dynamic is a separate option, independent of -z, so pass it as its own -Wl flag.

For consistency, pass the same flags to the external linker.

Thanks for @bcressey for having this fix already under review!

Testing done:
Built an aws-ecs-2-nvidia image and it boots correctly and uses the GPU

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[bcressey]

I wrote the commit so I'm not going to click "Approve", but I approve in spirit!

[yeazelm]

I wrote the commit so I'm not going to click "Approve", but I approve in spirit!

I figured but wanted to make sure you knew it was happening :)

[larvacea]

LDFLAGS are the worst. Just saying. Because of course -Wl,-z,relro -Wl