Avoid post-reloc GOT usage in __linker_init

A GOT lookup happening prior to soinfo::link_image causes a segfault. With
-O0, the compiler moves GOT lookups from after __linker_init's link_image
call to the start of __linker_init.

Rename the existing __linker_init_post_relocation to linker_main, then
extract the existing post-link_image code to a new
__linker_init_post_relocation function.

Bug: http://b/80503879
Test: /data/nativetest64/bionic-unit-tests/bionic-unit-tests
Test: manual
Change-Id: If8a470f8360acbe35e2a308b0fbff570de6131cf