RFR(XS): 8219675: Disable harfbuzz warnings with gcc 8

[Mikael Vidstedt]

Please review this change which (temporarily) disables two gcc warnings which are triggered by the harfbuzz code.

JBS: https://bugs.openjdk.java.net/browse/JDK-8219675 <https://bugs.openjdk.java.net/browse/JDK-8219675>
webrev: http://cr.openjdk.java.net/~mikael/webrevs/8219675/webrev.00/open/webrev/ <http://cr.openjdk.java.net/~mikael/webrevs/8219675/webrev.00/open/webrev/>

This change disables the two warnings in question (missing-attributes and class-memaccess). An alternative would be to make changes to the harfbuzz code itself instead.

Passes tier1.

Cheers,
Mikael