LoongArch: Use explicit relocs for addresses only used for one load or store...

LoongArch: Use explicit relocs for addresses only used for one load or store with -mexplicit-relocs=auto and -mcmodel={normal,medium}

In these cases, if we use explicit relocs, we end up with 2
instructions:

    pcalau12i    t0, %pc_hi20(x)
    ld.d         t0, t0, %pc_lo12(x)

If we use la.local pseudo-op, in the best scenario (x is in +/- 2MiB
range) we still have 2 instructions:

    pcaddi       t0, %pcrel_20(x)
    ld.d         t0, t0, 0

If x is out of the range we'll have 3 instructions.  So for these cases
just emit machine instructions with explicit relocs.

gcc/ChangeLog:

	* config/loongarch/predicates.md (symbolic_pcrel_operand): New
	predicate.
	* config/loongarch/loongarch.md (define_peephole2): Optimize
	la.local + ld/st to pcalau12i + ld/st if the address is only used
	once if -mexplicit-relocs=auto and -mcmodel=normal or medium.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/explicit-relocs-auto-single-load-store.c:
	New test.
	* gcc.target/loongarch/explicit-relocs-auto-single-load-store-no-anchor.c:
	New test.