riscv: kexec: do not add '-mno-relax' flag if compiler doesn't support it

The RISC-V special option '-mno-relax' which to disable linker relaxations
is supported by GCC8+. For GCC7 and lower versions do not support this
option.

Fixes: fba8a8674f ("RISC-V: Add kexec support")
Signed-off-by: Changbin Du <changbin.du@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/kernel/Makefile

@@ -11,7 +11,7 @@ endif
 CFLAGS_syscall_table.o	+= $(call cc-option,-Wno-override-init,)
 
 ifdef CONFIG_KEXEC
-AFLAGS_kexec_relocate.o := -mcmodel=medany -mno-relax
+AFLAGS_kexec_relocate.o := -mcmodel=medany $(call cc-option,-mno-relax)
 endif
 
 extra-y += head.o