bfd: use less memory in string merging

bfd: use less memory in string merging

the offset-to-entry mappings are allocated in blocks, which may
become a bit wasteful in case there are extremely many small
input files or sections.  This made it so that a large project
(Qt5WebEngine) didn't build anymore on x86 32bit due to address
space limits.  It barely fit into address space before the new
string merging, and then got pushed over the limit by this.

So instead of leaving the waste reallocate the maps to their final
size once known.  Now the link barely fits again.

bfd/
    * merge.c (record_section): Reallocate offset maps to their
    final size.