[AMDGPU][MC] Disable sendmsg SYSMSG_OP_HOST_TRAP_ACK on gfx9+ (#90203)

This is no longer supported as of gfx9. Fixes #52903

This commit also includes some refactoring of sendmsg operand parsing:
  - Use CustomOperand for sendmsg operations, this allows them to be
    conditionally available based on a STI check (and automatically in
    sync with SIDefines.h).
  - Move CustomOperand table lookups from AMDGPUBaseInfo to
    AMDGPUAsmUtils. This cleans up an awkward interface where
    AMDGPUAsmUtils defined a table/size as globals that AMDGPUBaseInfo
    had to loop over.
  - Clean up a few of the operand lookup functions while moving them.