RFR: JDK-8217723 Switch ld from bfd to gold on gcc toolchain

[Magnus Ihse Bursie]

The default binutils linker used by gcc, the bfd linker, is slow. The 
new replacement, gold, has been distributed alongside gcc for several 
years now, and is a well mature, and much faster, replacement.

This issue is about replacing ld.bfd with ld.gold for our gcc toolchain. 
In general, this cuts linking times by half or more.

As a future improvement, gold also allows for incremental linking. 
Turning on that is outside the scope of this issue.

Bug: https://bugs.openjdk.java.net/browse/JDK-8217723
Patch inline:
diff --git a/make/autoconf/flags-ldflags.m4 b/make/autoconf/flags-ldflags.m4
--- a/make/autoconf/flags-ldflags.m4
+++ b/make/autoconf/flags-ldflags.m4
@@ -70,7 +70,7 @@
      fi

      # Add -z defs, to forbid undefined symbols in object files.
-    BASIC_LDFLAGS="$BASIC_LDFLAGS -Wl,-z,defs"
+    BASIC_LDFLAGS="$BASIC_LDFLAGS -Wl,-z,defs -fuse-ld=gold"

      BASIC_LDFLAGS_JVM_ONLY="-Wl,-O1 -Wl,-z,relro"

/Magnus