RFR: [8u] 8141570: Fix Zero interpreter build for --disable-precompiled-headers

[Andrew John Hughes]

This is the first of a series of four changes to support -Wreturn-type
in OpenJDK 8u. The -Wreturn-type warning catches instances where control
flow exits a non-void function without returning a value. This can
combine with compiler optimisations in some cases to cause runtime
crashes. The warning is turned on by default in GCC 8 [0], causing the
build to fail if it is not explicitly disabled.

The subsequent changes are JDK-8062808 to apply the warning and fix
issues in the default build, JDK-8143245 to fix issues in the Zero build
and JDK-8197981 to fix one remaining Zero issue on 32-bit platforms only.

This change ensures the Zero build is not broken by 8062808 by disabling
-Wreturn-type in the Zero makefile.

Changes in the backport process:

* The G1 changes are dropped as 8u does not contain JDK-8073013, which
introduces g1EvacStats.*
* I've changed the ifeq ($(USE_CLANG), true) to ifeq
($(JVM_VARIANT_ZEROSHARK), true). There seems to have been a mistake in
the original patch which equated LLVM with clang. What is actually
referred to by "some version of llvm do not like -Wundef" is the LLVM
headers that Shark is built against, not the clang compiler which also
uses LLVM. This made its way through to the new HotSpot build in OpenJDK
9 & 10, but is gone as of JDK-8198724 in OpenJDK 11 and later.

This backport has been in IcedTea since 3.8.0 (2018-05-29) [1] and our
RHEL RPMs since 2018-06-16 without issue. Builds of current 8u-dev on
x86_64 with Zero release and Zero slowdebug succeeded.

[0] https://gcc.gnu.org/gcc-8/changes.html
[1] https://icedtea.classpath.org/bugzilla/show_bug.cgi?id=3548
-- 
Andrew :)

Senior Free Java Software Engineer
Red Hat, Inc. (http://www.redhat.com)

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