[llvm-objcopy] Make section rename/set flags case-insensitive

This fixes https://bugs.llvm.org/show_bug.cgi?id=41305. GNU objcopy
--set-section-flags/--rename-section flags are case-insensitive, so this
patch updates llvm-objcopy to match.

Reviewed by: grimar

Differential Revision: https://reviews.llvm.org/D60200

llvm-svn: 357590