RISC-V: Add -X to link spec

--discard-locals (-X) instructs the linker to remove local .L* symbols,
which occur a lot due to label differences for linker relaxation. The
arm port has a similar need and passes -X to ld.

In contrast, the RISC-V port does not pass -X to ld and rely on the
default --discard-locals in GNU ld's riscv port. The arm way is more
conventional (compiler driver instead of the linker customizes the
default linker behavior) and works with lld.

gcc/ChangeLog:

	* config/riscv/elf.h (LINK_SPEC): Add -X.
	* config/riscv/freebsd.h (LINK_SPEC): Add -X.
	* config/riscv/linux.h (LINK_SPEC): Add -X.

(cherry picked from commit 50c218e)

gcc/config/riscv/elf.h

@@ -20,6 +20,7 @@ along with GCC; see the file COPYING3.  If not see
 #define LINK_SPEC "\
 -melf" XLEN_SPEC DEFAULT_ENDIAN_SPEC "riscv \
 %{mno-relax:--no-relax} \
+-X \
 %{mbig-endian:-EB} \
 %{mlittle-endian:-EL} \
 %{shared}"

gcc/config/riscv/freebsd.h

@@ -44,6 +44,7 @@ along with GCC; see the file COPYING3.  If not see
   %{p:%nconsider using `-pg' instead of `-p' with gprof (1)}	\
   %{v:-V}							\
   %{assert*} %{R*} %{rpath*} %{defsym*}				\
+  -X								\
   %{mbig-endian:-EB}						\
   %{mlittle-endian:-EL}						\
   %{shared:-Bshareable %{h*} %{soname*}}			\

gcc/config/riscv/linux.h

@@ -50,6 +50,7 @@ along with GCC; see the file COPYING3.  If not see
 #define LINK_SPEC "\
 -melf" XLEN_SPEC DEFAULT_ENDIAN_SPEC "riscv" LD_EMUL_SUFFIX " \
 %{mno-relax:--no-relax} \
+-X \
 %{mbig-endian:-EB} \
 %{mlittle-endian:-EL} \
 %{shared} \