ld: Just xfail riscv little endian targets for compressed1d.d test.

The sizes of compressed and uncompressed .debug_aranges are the same
for the riscv big endian targets, but different for the little endian
targets.  The compress rule is fine and isn't broken by riscv, just the
original compressed1d.d fails by accident.  Ideally, we should fill the
R_RISCV_ADD/SUB relocations when disabling relaxations in the assembler.
But before that, Jim already had written an alternative test compressed1d-alt
to relpace compressed1d.d for riscv, so we can only xfail the riscv little
endian targets in the short-term.

ld/
    * testsuite/ld-elf/elf.exp (riscv_little_endian): Added.  Return true
    if target is riscv little endian.  Otherwise, return false.
    * testsuite/ld-elf/compressed1d.d: Only xfail the riscv little endian
    targets by [riscv_little_endian].