Store estimated istrances in compressed_size

Store estimated istrances in compressed_size

elf_x86_64_convert_load is very time consuming since it is called on
each input section and has a loop over input text sections to estimate
the branch distrance.  We can store the estimated distrances in the
compressed_size field of the output section, which is only used to
decompress the compressed input section.

Before the patch, linking clang 3.9 takes 52 seconds.  After the patch,
it only takes 2.5 seconds.

	PR ld/19542
	* elf64-x86-64.c (elf_x86_64_convert_load): Store the estimated
	distrances in the compressed_size field of the output section.