riscv: Use memcpy to handle unaligned access when fixing R_RISCV_RELATIVE

Although RISC-V Linux will enable the unaligned memory access handler by
default, that is quite expensive in general, using memcpy will be much cheaper
- just break down that into several load/store byte instructions.

ARM and MIPS has similar issue:

ARM: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51456
MIPS: https://gcc.gnu.org/legacy-ml/gcc-help/2005-07/msg00325.html

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

sysdeps/riscv/dl-machine.h

@@ -157,7 +157,10 @@ __attribute__ ((always_inline))
 elf_machine_rela_relative (ElfW(Addr) l_addr, const ElfW(Rela) *reloc,
 			  void *const reloc_addr)
 {
-  *(ElfW(Addr) *) reloc_addr = l_addr + reloc->r_addend;
+  /* R_RISCV_RELATIVE might located in debug info section which might not
+     aligned to XLEN bytes.  Also support relocations on unaligned offsets.  */
+  ElfW(Addr) value = l_addr + reloc->r_addend;
+  memcpy (reloc_addr, &value, sizeof value);
 }
 
 /* Perform a relocation described by R_INFO at the location pointed to