[XCOFF] support the overflow section (only relocation overflow is handled).

Summary: This patch handles relocation field overflows in an XCOFF32 file. (XCOFF64 files may not have overflow section headers.) If a section has more than 65,534 relocation entries or line number entries, both of these fields are set to a value of 65535. In this case, an overflow section header with the s_flags field equal to STYP_OVRFLO is used to contain the relocation and line-number count information. Since line number is not supported, this patch only handles the relocation overflow.

Reviewed By: shchenz

Differential Revision: https://reviews.llvm.org/D137819