RFR: JDK-8196998: Create devkit for Linux with gcc 7.3

[Tim Bell]

Erik:

> For oracle internal builds, we need to construct a portable devkit based
> on GCC 7.3. This change contains the updated makefile logic used to
> create this. The changes adds gdb and the gold linker. It also adds
> dynamic downloading of the sysroot rpms. Several long standing bugs were
> also fixed.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8196998
>
> Webrev: http://cr.openjdk.java.net/~erikj/8196998/webrev.01/


Looks good.

/Tim