LoongArch: Fix linker generate PLT entry for data symbol

LoongArch: Fix linker generate PLT entry for data symbol

With old "medium" code model, we call a function with a pair of PCALAU12I
and JIRL instructions. The assembler produces something like:

   8:	1a00000c 	pcalau12i   	$t0, 0
			8: R_LARCH_PCALA_HI20	g
   c:	4c000181 	jirl        	$ra, $t0, 0
			c: R_LARCH_PCALA_LO12	g

The linker generates a "PLT entry" for data without any diagnostic.
If "g" is a data symbol and ld with -shared option, it may load two
instructions in the PLT.

Without -shared option, loongarch_elf_adjust_dynamic_symbol can delete PLT
entry.

For R_LARCH_PCALA_HI20 relocation, linker only generate PLT entry for STT_FUNC
and STT_GNU_IFUNC symbols.