Add NS_LD variable to be used for alternate linker

[tdermendjiev]

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
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• All existing tests are passing: https://github.com/NativeScript/ios-runtime/blob/master/CONTRIBUTING.md#contribute-to-the-code-base
• Tests for the changes are included.

What is the current behavior?

nsld.sh script always calls $TOOLCHAIN_DIR/usr/bin/clang.

What is the new behavior?

Introduce new NS_LD variable that can point to an alternate linker that user wants to use. If not set - then we call the default one.

[Natalia-Hristova]

test

[Natalia-Hristova]

test