use hard-coded sh4a atomic opcodes to avoid linker errors on sh

use hard-coded sh4a atomic opcodes to avoid linker errors on sh

when using the sh4a opcodes, the assembler tags the resulting object
file as requiring sh4a. the linker then refuses to (static) link it
with object files marked as requiring j2, since there is no isa level
that includes both sh4a and j2 instructions.