arm64: Add support to read symbols like _stext from '/proc/kallsyms'

On ARM64 platforms the VA_BITS supported by a linux kernel being run
can be selected by setting 'ARM64_VA_BITS_*' (see 'arch/arm64/Kconfig'
for details).

Now, to determine the 'info->page_offset' in arm64 makedumpfile
context ('arch/arm64.c') we need to determine the VA_BITS which was
selected by the underlying linux kernel.

There can be several ways to determine the VA_BITS, out of which
reading the '_stext' symbol and calculating the 'va_bits' using the
same is the simplest yet portable method.

For reading the kernel symbols like '_stext', we can read the
'/proc/kallsyms' file which contains these symbols and
works even in case of KASLR enabled arm64 kernels, as in those cases,
the '_stext' symbol will end up being randomized and hence
cannot be correctly read from the 'vmlinux' file.

Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>