Allow ELF section flags to be specified numerically

Summary:
GAS already allows flags for sections to be specified directly as a
numeric value. This functionality is particularly useful for setting
processor or application-specific values that may not be directly
supported or understood by LLVM. This patch allows LLVM to use numeric
section flag values verbatim if specified by the assembly file.

Reviewers: grosbach, rafael, t.p.northover, rengolin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D27451

llvm-svn: 289785