[8u] RFR: 8262730: Enable jdk8u MacOS external debug symbols

[Andrew Leonard]

Hi,
Please can get reviews for a patch to enable jdk8u MacOS external debug
symbols, which we have already applied at AdoptOpenJDK, and would like to
contribute back upstream.
wbrev: https://cr.openjdk.java.net/~aleonard/8262730/webrev.00/
bug: https://bugs.openjdk.java.net/browse/JDK-8262730

Thank you,
Andrew