[llvm-readobj] - Stop treating ".stack_sizes.*" sections as stack sizes sections.

llvm-readobj currently handles .stack_sizes.* (e.g. .stack_sizes.foo)
as a normal stack sizes section. Though MC does not produce sections with
such names. Also, linkers do not combine .stack_sizes.* into .stack_sizes.

A mini discussion about this correctness issue is here: https://reviews.llvm.org/D67757#inline-609274
This patch changes implementation so that only now only '.stack_sizes' name is
accepted as a real stack sizes section.

Differential revision: https://reviews.llvm.org/D67824

llvm-svn: 372578