[demangler][LoongArch] Correct the mangled_size for long double

The size of long double in LoongArch (both LA32 and LA64) is 16 bytes, thus
the mangled_size should be 32.

This is same as RISCV's change in D126480.

Reviewed By: xen0n

Differential Revision: https://reviews.llvm.org/D138981