builtins: cleanup constant data section selection

Each of the object formats use a different directive for selecting the constant
section.  Use a macro to avoid the duplication across a number of files.  Also
correct a small macro mismatch on the Windows case (HIDDEN_DIRECTIVE -> HIDDEN).

Patch by Vadim Chugunov!

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@223910 91177308-0d34-0410-b5e6-96231b3b80d8