libreoffice: add "CXXFLAGS+= -march=i586" on i386 for 64bit atomic ops.

Fixes the first part of PR/54826.  libreoffice-6.4.2.2 from
pkgsrc-2020Q1 on NetBSD/i386 9.0 builds and works with this change.
Note the second part of the PR (memalign error) was already fixed as
patches/patch-external_pdfium_pkgsrc2.patch.1 in the following commit:
 https://mail-index.netbsd.org/pkgsrc-changes/2020/02/07/msg206251.html

misc/libreoffice/Makefile

@@ -1,4 +1,4 @@
-# $NetBSD: Makefile,v 1.225 2020/04/18 04:18:05 ryoon Exp $
+# $NetBSD: Makefile,v 1.226 2020/04/18 11:16:31 tsutsui Exp $
 
 VERREL=		6.4.3
 VERRC=		2
@@ -252,6 +252,11 @@ BUILD_MAKE_FLAGS+=	VERBOSE=true
 BUILDLINK_TRANSFORM+=	rm:-ldl
 .endif
 
+.if ${MACHINE_ARCH} == "i386"
+# For 64 bit atomic ops (PR/54826)
+CXXFLAGS+=		-march=i586
+.endif
+
 INSTALLATION_DIRS=	bin share/applications
 
 .PHONY: debugrun