ld.bfd requires an explicit emulation for MIPS for ld -r.

Unlike lld, ld.bfd doesn't infer the emulation from the first object
file, but assumes its compiled in default for ld -r.

Differential Revision:	https://reviews.freebsd.org/D25728

lib/csu/mips/Makefile

@@ -2,6 +2,27 @@
 
 .PATH: ${.CURDIR:H}/common
 
+.include <src.opts.mk>
+
 CFLAGS+=	-DCRT_IRELOC_SUPPRESS
 
+.if ${MACHINE_ARCH:Mmips64}
+ELFCLASS=	64
+.else
+ELFCLASS=	32
+.endif
+.if ${MACHINE_ARCH:Mmips*el}
+ENDIAN=		l
+.else
+ENDIAN=		b
+.endif
+.if ${MACHINE_ARCH:Mmipsn32*}
+SUFFIX=		n32
+.else
+SUFFIX=
+.endif
+
+# binutils requires an explicit emulation for ld -r
+LDFLAGS.bfd+=	-Wl,-m -Wl,elf${ELFCLASS}${ENDIAN}tsmip${SUFFIX}_fbsd
+
 .include <bsd.lib.mk>