RFR: JDK-8170784: JDK-8170651 broke cross compilation

[Erik Joelsson]

When cleaning up legacy hotspot settings in JDK-8170651, a bit too much 
was removed. Now we can't cross compile certain arm platforms.

Bug: https://bugs.openjdk.java.net/browse/JDK-8170784

Patch:

diff -r 9475c1488d29 common/autoconf/buildjdk-spec.gmk.in
--- a/common/autoconf/buildjdk-spec.gmk.in
+++ b/common/autoconf/buildjdk-spec.gmk.in
@@ -89,4 +89,10 @@
  # Control wether Hotspot builds gtest tests
  BUILD_GTEST := false

JVM_VARIANTS := server
+
+# Some users still set EXTRA_*FLAGS on the make command line. Must
+# make sure to override that when building buildjdk.
+override EXTRA_CFLAGS :=
+override EXTRA_CXXFLAGS :=
+override EXTRA_LDFLAGS :=


/Erik