clean up mips (32-bit, o32) syscall asm constraints

clean up mips (32-bit, o32) syscall asm constraints

analogous to commit ddc7c4f936c7a90781072f10dbaa122007e939d0 for
mips64 and n32, remove the hack to load the syscall number into $2 via
asm, and use a constraint to let the compiler load it instead.

now, only $4, $5, and $6 are potential input-only registers. $2 is
always input and output, and $7 is both when it's an argument,
otherwise output-only. previously, $7 was treated as an input (with a
"1" constraint matching its output position) even when it was not an
input, which was arguably undefined behavior (asm input from
indeterminate value). this is corrected.

as before, $8, $9, and $10 are conditionally input-output registers
for 5-, 6-, and 7-argument syscalls. their role in input is carrying
in the values that will be stored on the stack for arguments 5-7.
their role in output is carrying back whatever the kernel has
clobbered them with, so that the compiler cannot assume they still
contain the input values.