MIPS: fix r6 testsuites

Introduce
	run_dump_test_o32l
	run_dump_test_n32l
	run_dump_test_n64l
Which use `-march=from-abi` for pre-R6 testcases,
like micromips/mips16e etc.

For cases doesn't use run_dump_test_*, we use
	-mips32r2 for micromips32
	-mips1 for mips16-32
	-march=from-abi for testcases to o32/n32/n64 both/all.

Replace `addi` with `addiu` for some cases for both r6 and pre-R6.

Introduce some new testcases for r6 with FPXX/FP64.
Introduce new testcase: comdat-reloc-r6.

Skip `default` in mips_arch_list_matching if triple is mipsisa*, due to:
  1)it will cannot match mipsr6@*.d: since mips32rN/mips64rN
    will always be used, it won't be a problem.
  2)some test think -march=mips64rN will alway true for mipsisa64rN,
    which is not true now.

This patch fix testsuite for all r6-default gnu triples:
  mipsisa32r6-linux-gnu
  mipsisa32r6el-linux-gnu
  mips-img-linux-gnu
  mipsel-img-linux-gnu
  mipsisa64r6-linux-gnu
  mipsisa64r6el-linux-gnu