[ELF] Allow output section data commands to take expressions

The current implementation of the output section data store commands
can only handle integer literals, but it should really handle arbitrary
expressions [1].  This commit fixes that.

[1] https://sourceware.org/binutils/docs-2.27/ld/Output-Section-Data.html#Output-Section-Data

Differential Revision: https://reviews.llvm.org/D27561

llvm-svn: 289152