Convert the load_primitive_string_utf16_code_unit macro to take its
inputs (string pointer, index) and output (code_unit) as explicit
parameters, plus migrate CallBuiltinStringFromCharCode,
CallBuiltinStringPrototypeCharCodeAt, and
CallBuiltinStringPrototypeCharAt.
The macro previously hard-coded inputs to t2/t4 and output to t0,
which forced callers to remember those slot assignments and to wrap
the call_helper interactions around them. The migrated form -- where
the caller names what it wants in each slot -- removes another
"clobbers t3, t5" comment.
f816e43fbb