RFR (8u): 8034788: Rewrite toolchain.m4 to support multiple toolchains per platform

[Kevin Walls]

Hi,

I'd like to get a technical review of a backport to jdk8u:

8034788: Rewrite toolchain.m4 to support multiple toolchains per platform
JBS: https://bugs.openjdk.java.net/browse/JDK-8034788
9 changeset: http://hg.openjdk.java.net/jdk9/jdk9/rev/b18872ff5379

8u webrev: http://cr.openjdk.java.net/~kevinw/8034788/webrev.00/

toolchain.m4 did not import, lots of manual changes there, most other 
files imported OK.

This change adds flags.m4 which we have in 9, but not in 8 until now. 
When 8151841 happened in 8u ( 
http://closedjdk.us.oracle.com/jdk8u/jdk8u-cpu/rev/73494e6ff8e5 ) it 
worked with toolchain.m4, and didn't create flags.m4.  But doing 8034788 
now, I wanted to bring 8 and 9 closer together, to make further changes 
easier, so I add flags.m4, and e.g. FLAGS_SETUP_GCC6_COMPILER_FLAGS 
moves to flags.m4.

(I updated copyright years after creating the webrev.)

Thanks
Kevin