[demangler] PPC and S390: Fix parsing of e-prefixed long double literals

Summary:
This patch is to fix the parsing of long double literals encoded with the e prefix on PowerPC and S390. For both PowerPC and S390, type code e is used for 64-bit long double literals and g is used for 128-bit long double literals. libcxxabi test case test_demangle.pass.cpp fails without the fix.

Authored by: xingxue-ibm

Reviewers: hubert.reinterpretcast, jasonliu, erik.pilkington, uweigand, mclow.li
sts, libc++abi

Reviewed by: hubert.reinterpretcast, erik.pilkington

Differential Revision: https://reviews.llvm.org/D74163