[BOLT] Don't abort on processing binaries with .gdb_index section

Summary:
While writing non-allocatable sections we had an assumption that the
size of such section is congruent to the alignment, as typically
such sections are a collections of fixed-sized elements. .gdb_index
breaks this assumption.

This diff removes the assertion that was triggered by a presence of
.gdb_index section, and makes sure that we insert a padding if we are
appending to a section with a size not congruent to section alignment.

(cherry picked from FBD4844553)