linker: find AT_BASE using AT_PHDR/AT_PHNUM

When the linker is invoked directly, rather than as an interpreter for a
real program, the AT_BASE value is 0. To find the linker's base address,
the linker currently relies on the static linker populating the target of
a RELA relocation with an offset rather than leaving it zero. (With lld,
it will require a special flag, --apply-dynamic-relocs.)

Instead, do something more straightforward: the linker already finds the
executable's base address using its PHDR table, so do the same thing when
the linker is run by itself.

Bug: http://b/72789859
Test: boots, run linker/linker64 by itself
Change-Id: I4da5c346ca164ea6f4fbc011f8c3db4e6a829456