[BOLT] Set IsRelro section attribute based on PT_GNU_RELRO segment

Handle PT_GNU_RELRO segment in accordance with Linux Standard Base spec
chapter 12:

> PT_GNU_RELRO
> The array element specifies the location and size of a segment which may
> be made *read-only* after relocations have been processed.

Perform a readelf-style mapping check between this segment and sections,
set `IsRelro` section attribute.

Reviewed By: #bolt, maksfb

Differential Revision: https://reviews.llvm.org/D152944