[demangler][RISCV] Fix for long double

Summary:
The size of long double in RISCV (both RV32 and RV64) is 16 bytes, thus
the mangled_size shouble be 32.

This patch will fix test case
"_ZN5test01hIfEEvRAcvjplstT_Le4001a000000000000000E_c"
in test_demangle.pass.cpp, which is expected to be invalid but demangler
returned "void test0::h<float>(char (&) [(unsigned int)((sizeof (float))
+ (0x0.000000004001ap-16382L))])" in RISCV environment without this patch.

Reviewed By: urnathan

Differential Revision: https://reviews.llvm.org/D126480