Move the dynamic linker to ./bootstrap subdir

The dynamic linker installed to /system partition is only for
bootstraping, i.e. to be used only by early processes. The existing path
/system/bin/linker[64] will be a symlink to the mount pointpoint
/bionic/bin/linker[64] which will be bind-mounted with either the
bootstrap linker or the default linker in the runtime APEX. Since the
path is taken by the symlink, bootstrap linker is moved to ./boostrap
subdirectory.

Bug: 120266448
Test: m linker; the linker is found under /system/bin/bootstrap/
Change-Id: I163e0c8a1738b94565a6b60f48e65841ba058a91