linux/linkage.h: replace VMLINUX_SYMBOL_STR() with __stringify()

With the special case handling for Blackfin and Metag was removed by
commit 94e58e0a ("export.h: remove code for prefixing symbols with
underscore"), VMLINUX_SYMBOL_STR() is now equivalent to __stringify().

Replace the remaining usages in <linux/linkage.h> to prepare for the
entire removal of VMLINUX_SYMBOL_STR().

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