modpost: add a helper to get data pointed by a symbol

When CONFIG_MODULE_REL_CRCS is enabled, the value of __crc_* is not
an absolute value, but the address to the CRC data embedded in the
.rodata section.

Getting the data pointed by the symbol value is somewhat complex.
Split it out into a new helper, sym_get_data().

I will reuse it to refactor namespace_from_kstrtabns() in the next
commit.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>