RFR: JDK-8190284 link-time-opt should not be using -fwhole-program

[Magnus Ihse Bursie]

When building hotspot with link-time-opt, we are currently using 
-fwhole-program to the linker. This causes all public symbols to be 
removed. To counter this, the attribute "externally_visible" has been 
added to JNIEXPORT for arm (the platform that traditionally has used 
link-time-opt).

This is not an appropriate way of doing things.

The gcc documentation says about "-fwhole-program": "This option should 
not be used in combination with -flto. Instead relying on a linker 
plugin should provide safer and more precise information."

If we replace -fwhole-program with -fuse-linker-plugin, the need to use 
externally_visible disappears.

I intend to push this to jdk10/hs. This is a blocker for JDK-8189610.

I have verified that with this change I can build a link-time-opt 
libjvm.so on my linux-x64 workstation, that passes superficial tests, 
and that is ~4% smaller in size than a non-lto build.

Bug: https://bugs.openjdk.java.net/browse/JDK-8190284
Patch inline:
diff --git a/make/hotspot/lib/JvmFeatures.gmk 
b/make/hotspot/lib/JvmFeatures.gmk
--- a/make/hotspot/lib/JvmFeatures.gmk
+++ b/make/hotspot/lib/JvmFeatures.gmk
@@ -157,7 +157,7 @@
    # like the old build, but it's probably not right.
    JVM_OPTIMIZATION :=
    JVM_CFLAGS_FEATURES += -O3 -flto
-  JVM_LDFLAGS_FEATURES += -O3 -flto -fwhole-program -fno-strict-aliasing
+  JVM_LDFLAGS_FEATURES += -O3 -flto -fuse-linker-plugin 
-fno-strict-aliasing
  endif

  ifeq ($(call check-jvm-feature, minimal), true)

/Magnus