ld128 demangle: allow space for 'L' suffix.

Summary:
Caught by HWASAN on arm64 Android (which uses ld128 for long double). This
was running the existing fuzzer.

The specific minimized fuzz input to reproduce this is:

  __cxa_demangle("1\006ILeeeEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE", 0, 0, 0);

Reviewers: eugenis, srhines, #libc_abi!

Subscribers: kristof.beyls, danielkiss, libcxx-commits

Tags: #libc_abi

Differential Revision: https://reviews.llvm.org/D77924