LoongArch: Add new relocs and macro for TLSDESC.

The normal DESC instruction sequence is:
  pcalau12i  $a0,%desc_pc_hi20(var)     #R_LARCH_TLS_DESC_PC_HI20
  addi.d     $a0,$a0,%desc_pc_lo12(var) #R_LARCH_TLS_DESC_PC_LO12
  ld.d       $ra,$a0,%desc_ld(var)	#R_LARCH_TLS_DESC_LD
  jirl       $ra,$ra,%desc_call(var)	#R_LARCH_TLS_DESC_CALL
  add.d	     $a0,$a0,$tp