[8] RFR(XXS) 8203845: backport of JDK-8034788 inadvertently rolled back JDK-8187045 changes to toolchain.m4

[David Buck]

Hi!

Please review this very simple fix. The backport for JDK-8034788 [0] did 
a lot of refactoring and moved the some code from toolchain.m4 to 
flags.m4. Unfortunately, it also unintentionally rolled back my backport 
of JDK-8187045 [1] when it moved this chunk of code. As a result, this 
is only an issue in JDK 8.

All I did in this fix was reapply the same change that I had previously 
made to toolchain.m4 for JDK- 8187045 [2], but to flags.m4 where the 
relevant code now lives.

bug report:
https://bugs.openjdk.java.net/browse/JDK-8203845

webrev:
http://cr.openjdk.java.net/~dbuck/8203845.0/

Everything has been built and tested (JPRT, manual confirmation of fix). 
Of course I will also push the closed source version of 
generated-configure.sh with this fix.

Cheers,
-Buck

[0] https://bugs.openjdk.java.net/browse/JDK-8034788
[1] https://bugs.openjdk.java.net/browse/JDK-8187045
[2] http://hg.openjdk.java.net/jdk8u/jdk8u-dev/rev/c0b1ccecf70f