[llvm-objcopy] Add --prefix-alloc-sections

[llvm-objcopy] Add --prefix-alloc-sections

This patch adds support for --prefix-alloc-sections, which adds a prefix
to every allocated section names.

It adds a prefix after renaming section names by --rename-section as GNU
objcopy does.

Fixes PR41266: https://bugs.llvm.org/show_bug.cgi?id=41266

Differential Revision: https://reviews.llvm.org/D60042

Patch by Seiya Nuta.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@360233 91177308-0d34-0410-b5e6-96231b3b80d8