Add a new "riscv-relaxations" linker feature.

When the linker doesn't have this feature, add -mno-relax to CFLAGS
on RISC-V.

Define the feature for ld.bfd, but not lld.  If lld gains relaxation
support in a newer version, we can enable it for those versions of lld
in bsd.linker.mk.

Reviewed by:	mhorne
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D22659

share/mk/bsd.cpu.mk

@@ -375,6 +375,10 @@ CFLAGS += -march=rv64imac -mabi=lp64
 .else
 CFLAGS += -march=rv64imafdc -mabi=lp64d
 .endif
+
+.if ${LINKER_FEATURES:U:Mriscv-relaxations} == ""
+CFLAGS += -mno-relax
+.endif
 .endif
 
 # NB: COPTFLAGS is handled in /usr/src/sys/conf/kern.pre.mk

share/mk/bsd.linker.mk

@@ -86,6 +86,9 @@ ${X_}LINKER_FEATURES=
 ${X_}LINKER_FEATURES+=	build-id
 ${X_}LINKER_FEATURES+=	ifunc
 .endif
+.if ${${X_}LINKER_TYPE} == "bfd" && ${${X_}LINKER_VERSION} > 21750
+${X_}LINKER_FEATURES+=	riscv-relaxations
+.endif
 .if ${${X_}LINKER_TYPE} == "lld" && ${${X_}LINKER_VERSION} >= 60000
 ${X_}LINKER_FEATURES+=	retpoline
 .endif

sys/conf/kern.mk

@@ -144,6 +144,10 @@ CFLAGS+=	-march=rv64imafdc -mabi=lp64
 CFLAGS.clang+=	-mcmodel=medium
 CFLAGS.gcc+=	-mcmodel=medany
 INLINE_LIMIT?=	8000
+
+.if ${LINKER_FEATURES:Mriscv-relaxations} == ""
+CFLAGS+=	-mno-relax
+.endif
 .endif
 
 #