kallsyms: remove unused arch_get_kallsym() helper

The arch_get_kallsym() function was introduced so that x86 could override
it, but that override was removed in bf904d27 ("x86/pti/64: Remove
the SYSCALL64 entry trampoline"), so now this does nothing except causing
a warning about a missing prototype:

kernel/kallsyms.c:662:12: error: no previous prototype for 'arch_get_kallsym' [-Werror=missing-prototypes]
  662 | int __weak arch_get_kallsym(unsigned int symnum, unsigned long *value,

Restore the old behavior before d83212d5 ("kallsyms, x86: Export
addresses of PTI entry trampolines") to simplify the code and avoid
the warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Alan Maguire <alan.maguire@oracle.com>
[mcgrof: fold in bpf selftest fix]
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>