MIPS: vmlinux.lds.S: explicitly catch .rel.dyn symbols

According to linker warnings, both GCC and LLVM generate '.rel.dyn'
symbols:

mips-alpine-linux-musl-ld: warning: orphan section `.rel.dyn'
from `init/main.o' being placed in section `.rel.dyn'

Link-time assertion shows that this section is sometimes empty,
sometimes not, depending on machine bitness and the compiler [0]:

  LD      .tmp_vmlinux.kallsyms1
mips64-linux-gnu-ld: Unexpected run-time relocations (.rel) detected!

Just use the ARM64 approach and declare it in vmlinux.lds.S closer
to __init_end.

[0] https://lore.kernel.org/linux-mips/20210109111259.GA4213@alpha.franken.de

Reported-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/kernel/vmlinux.lds.S

@@ -137,6 +137,11 @@ SECTIONS
 	PERCPU_SECTION(1 << CONFIG_MIPS_L1_CACHE_SHIFT)
 #endif
 
+	.rel.dyn : ALIGN(8) {
+		*(.rel)
+		*(.rel*)
+	}
+
 #ifdef CONFIG_MIPS_ELF_APPENDED_DTB
 	.appended_dtb : AT(ADDR(.appended_dtb) - LOAD_OFFSET) {
 		*(.appended_dtb)