[flang] Add LoongArch64 support to lib/Optimizer/CodeGen/Target.cpp

Add LoongArch64 linux target specifics to Target.cpp which is similar to
RISCV-64 in D136547.

For LoongArch, a complex floating-point number, or a structure
containing just one complex floating-point number, is passed as though
it were a structure containing two floating-point reals.

Reviewed By: vzakhari

Differential Revision: https://reviews.llvm.org/D143131