MIPS: Add conditions for use of the -mmips16e2 and -mips16 option.

MIPS: Add conditions for use of the -mmips16e2 and -mips16 option.

Changes from V1:
* Raise the minimal revision to r2.

MIPS16e2 ASE is a superset of MIPS16e ASE, which is again a superset
of MIPS16 ASE. Later, all of them are forbidden in Release 6.

Make -mmips16e2 imply -mips16 as the ASE requires, so users won't
be surprised even if they expect it to. Meanwhile, check if
mips_isa_rev <= 5 when -mips16 is effective and >= 2 when -mmips16e2
is effective.

Co-developed-by: Rong Zhang <rongrong@oss.cipunited.com>
Signed-off-by: Rong Zhang <rongrong@oss.cipunited.com>
gcc/ChangeLog:
	* config/mips/mips.cc(mips_option_override):Add conditions
	for use of the -mmips16e2 and -mips16 option.

gcc/testsuite/ChangeLog:
	* gcc.target/mips/mips16e2-cache.c: Use isa_rev>=2 instead of
	-mips32r2 and remove -mips16 option.
	* gcc.target/mips/mips16e2-cmov.c: Add isa_rev>=2 and remove
	-mips16 option.
	* gcc.target/mips/mips16e2-gp.c: Same as above.
	* gcc.target/mips/mips16e2.c: Same as above.