LoongArch: Warn about right shifts of negative numbers

LoongArch: Warn about right shifts of negative numbers

The GNU Assembler User Guide says that the right shift operator ">>"
in an expression is the same as the C operator.

On LoongArch the assembler directives and instructions do not treat
negative numbers ">>" the same way. The directives treats negative
numbers ">>" as logical right shifts while the instructions treats them
as arithmetic right shifts.

The right shift of negative numbers in the instructions may be changed
from an arithmetic right shift to a logical right shift in the future,
and a warning is issued for this.