java/openjdk8: fix build with clang 16

Since clang 16 (and gcc 11) the default C++ standard is now gnu++17.
Because java/openjdk8's build infrastructure does not explicitly set its
C++ standard, this leads to several errors:

  /wrkdirs/usr/ports/java/openjdk8/work/jdk8u-jdk8u362-b09.1/hotspot/src/share/vm/adlc/arena.cpp:82:3: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
    register Chunk *k = _first;
    ^~~~~~~~~
  /wrkdirs/usr/ports/java/openjdk8/work/jdk8u-jdk8u362-b09.1/hotspot/src/share/vm/adlc/arena.cpp:96:3: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
    register Chunk *k = _chunk;   // Get filled-up chunk address
    ^~~~~~~~~
  2 errors generated.
  /wrkdirs/usr/ports/java/openjdk8/work/jdk8u-jdk8u362-b09.1/hotspot/src/share/vm/adlc/adlparse.cpp:4567:3: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
    register char c;
    ^~~~~~~~~
  /wrkdirs/usr/ports/java/openjdk8/work/jdk8u-jdk8u362-b09.1/hotspot/src/share/vm/adlc/adlparse.cpp:4765:3: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
    register char c;
    ^~~~~~~~~

For newer versions of OpenJDK this has been fixed by removing the
'register' keyword, but upstream has not yet backported it to OpenJDK 8.
To work around the errors, define the register keyword away using a
command line flag.

PR:		271080
Approved by:	glewis (maintainer)
MFH:		2023Q2

java/openjdk8/Makefile

@@ -1,5 +1,6 @@
 PORTNAME=	openjdk
 PORTVERSION=	${JDK_MAJOR_VERSION}.${JDK_UPDATE_VERSION}.${JDK_BUILD_NUMBER}.${BSD_JDK_VERSION}
+PORTREVISION=	1
 CATEGORIES=	java devel
 MASTER_SITES=	LOCAL/jkim:jtreg
 PKGNAMESUFFIX?=	${JDK_MAJOR_VERSION}
@@ -213,6 +214,13 @@ BUILD_DEPENDS+=	clang${LLVM_VER}:devel/llvm${LLVM_VER}
 CC=		${LOCALBASE}/bin/clang${LLVM_VER}
 CXX=		${LOCALBASE}/bin/clang++${LLVM_VER}
 .endif
+.if ${COMPILER_VERSION} >= 160
+# clang 16 defaults to C++17, which no longer allows the 'register' keyword.
+# There is an upstream commit that removes all the individual 'register'
+# keywords, but it has not yet been backported to OpenJDK 8.
+# NOTE: passing this option via --with-extra-cflags does not work.
+CFLAGS+=		-Dregister=
+.endif
 .endif
 
 # GCC is broken with PCH: https://lists.freebsd.org/pipermail/svn-src-all/2015-March/101722.html