* Fix demangling template parameter packs

Clang 4.0.1-10 and gcc 7.3.0 both mangle the function "void add<int>(int)" as
"_Z3addIJiEEvDpT_".  The template parameter pack is of the form
J <template-arg>* E

The opening character for a param pack could be either I or J, as libiberty
follows [1].  This change simply adds the J case.

[1] https://github.com/gcc-mirror/gcc/blob/fbd263526ad105a953fd51d9f7bca2c3f268cf82/libiberty/cp-demangle.c#L3209