Remove the XCore custom implementation of MergeableConstSection, relying on

Remove the XCore custom implementation of MergeableConstSection, relying on
the generic ELF version instead.  This will result in its mergable constant
sections getting named ".rodata.cst4" instead of ".cp.const4", but the
linker looks at the section flags, not the name of the section AFAICT.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@76659 91177308-0d34-0410-b5e6-96231b3b80d8