MIPS/testsuite: Expand GAS and LD HI/LO relocation coverage

Expand test coverage for HI/LO relocation handling and add conventional
MIPS and microMIPS GAS tests as well as conventional MIPS, microMIPS,
and MIPS16e2 LD tests, covering R_MIPS_HI16, R_MIPS_LO16, R_MIPS16_HI16,
R_MIPS16_LO16, R_MICROMIPS_HI16, and R_MICROMIPS_LO16 relocations, as
well as 64-bit R_MIPS_HIGHEST, R_MIPS_HIGHER, R_MICROMIPS_HIGHEST, and
R_MICROMIPS_HIGHER relocations.

Modify the linker script so as to retain the `.MIPS.abiflags' section so
as to disassemble MIPS16e2 code correctly, as MIPS16e2 ASE information
is only carried in that section and not in ELF file header's `e_flags'.

MIPS16e2 and microMIPS code requires at least the MIPS32r2 ISA (or the
MIPS64r2 one for the n32 and n64 ABIs), which is incompatible with the
`mips:5900' linker output architecture and causes link failures such as:

./ld-new: tmpdir/mips-hilo1.o: linking mips:isa32r2 module with previous mips:5900 modules
./ld-new: failed to merge target specific data of file tmpdir/mips-hilo1.o

Therefore exclude `mips*el-ps2-elf*' targets from microMIPS and MIPS16e2
LD testing.