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

[Erik Joelsson]

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/

/Erik