[mips] fix up the assembly generation of unaligned exception loads

[mips] fix up the assembly generation of unaligned exception loads

I noticed that unaligned accesses were returning garbage values.

Give test data like this:

char testdata[] = { 0x12, 0x34, 0x56, 0x78, 0x9a, 0xbc, 0xde, 0xf1, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef, 0x5a };

Iterating through uint32_t space 1 byte at a time should
look like this:

freebsd-carambola2:/mnt# ./test
Hello, world!
offset 0 pointer 0x410b00 value 0x12345678 0x12345678
offset 1 pointer 0x410b01 value 0x3456789a 0x3456789a
offset 2 pointer 0x410b02 value 0x56789abc 0x56789abc
offset 3 pointer 0x410b03 value 0x789abcde 0x789abcde
offset 4 pointer 0x410b04 value 0x9abcdef1 0x9abcdef1
offset 5 pointer 0x410b05 value 0xbcdef123 0xbcdef123
offset 6 pointer 0x410b06 value 0xdef12345 0xdef12345
offset 7 pointer 0x410b07 value 0xf1234567 0xf1234567

.. but to begin with it looked like this:

offset 0 value 0x12345678
offset 1 value 0x00410a9a
offset 2 value 0x00419abc
offset 3 value 0x009abcde
offset 4 value 0x9abcdef1
offset 5 value 0x00410a23
offset 6 value 0x00412345
offset 7 value 0x00234567

The amusing reason? The compiler is generating the lwr/lwl incorrectly.
Here's an example after I tried to replace the two macros with a single
invocation and offset, rather than having the compiler compile in addiu
to s3 - but the bug is the same:

1044: 8a620003 lwl v0,0(s3)
1048: 9a730000 lwr s3,3(s3)

.. which is just totally trashy and wrong.

This explicitly tells the compiler to treat the output as being read
and written to, which is what lwl/lwr does with the destination
register.

I think a subsequent commit should unify these macros to skip an addiu,
but that can be a later commit.

Reviewed by:	jhb
Differential Revision:	https://reviews.freebsd.org/D25040