[llvm-readobj] - Teach tool to dump objects with >= SHN_LORESERVE of sections.

http://www.sco.com/developers/gabi/2003-12-17/ch4.eheader.html

says that e_shnum and/or e_shstrndx may have special values if
"the number of sections is greater than or equal to SHN_LORESERVE" or
"the section name string table section index is greater than or equal to SHN_LORESERVE (0xff00)"

Previously llvm-readobj was unable to dump such files, patch changes that.

I had to add a precompiled test case because it does not seem possible to
prepare a test using yaml2obj or llvm-mc (not clear how to make .shstrtab
to have index >= SHN_LORESERVE).

Differential revision: https://reviews.llvm.org/D49369

llvm-svn: 337360