elf: Fix GL(dl_phdr) and GL(dl_phnum) for static builds [BZ #29864]

elf: Fix GL(dl_phdr) and GL(dl_phnum) for static builds [BZ #29864]

The 73fc4e28b9464f0e refactor did not add the GL(dl_phdr) and
GL(dl_phnum) for static build, relying on the __ehdr_start symbol,
which is always added by the static linker, to get the correct values.

This is problematic in some ways:

  - The segment may see its in-memory size differ from its in-file
    size (or the binary may have holes).  The Linux has fixed is to
    provide concise values for both AT_PHDR and AT_PHNUM (commit
    0da1d5002745c - "fs/binfmt_elf: Fix AT_PHDR for unusual ELF files")

  - Some archs (alpha for instance) the hidden weak reference is not
    correctly pulled by the static linker and  __ehdr_start address
    end up being 0, which makes GL(dl_phdr) and GL(dl_phnum) have both
    invalid values (and triggering a segfault later on libc.so while
    accessing TLS variables).

The safer fix is to just restore the previous behavior to setup
GL(dl_phdr) and GL(dl_phnum) for static based on kernel auxv.  The
__ehdr_start fallback can also be simplified by not assuming weak
linkage (as for PIE).

The libc-static.c auxv init logic is moved to dl-support.c, since
the later is build without SHARED and then GLRO macro is defined
to access the variables directly.

The _dl_phdr is also assumed to be always non NULL, since an invalid
NULL values does not trigger TLS initialization (which is used in
various libc systems).

Checked on aarch64-linux-gnu, x86_64-linux-gnu, and i686-linux-gnu.

Reviewed-by: Florian Weimer <fweimer@redhat.com>