vmcore: Remove "weak" from function declarations

For the following functions:

  elfcorehdr_alloc()
  elfcorehdr_free()
  elfcorehdr_read()
  elfcorehdr_read_notes()
  remap_oldmem_pfn_range()

fs/proc/vmcore.c provides default definitions explicitly marked "weak".
arch/s390 provides its own definitions intended to override the default
ones, but the "weak" attribute on the declarations applied to the s390
definitions as well, so the linker chose one based on link order (see
10629d71 ("PCI: Remove __weak annotation from pcibios_get_phb_of_node
decl")).

Remove the "weak" attribute from the declarations so we always prefer a
non-weak definition over the weak one, independent of link order.

Fixes: be8a8d06 ("vmcore: introduce ELF header in new memory feature")
Fixes: 9cb21813 ("vmcore: introduce remap_oldmem_pfn_range()")
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
CC: Michael Holzheu <holzheu@linux.vnet.ibm.com>