ARC: fix CONFIG_HARDENED_USERCOPY

Currently enabling this triggers a warning

| usercopy: Kernel memory overwrite attempt detected to kernel text (offset 155633, size 11)!
| usercopy: BUG: failure at mm/usercopy.c:99/usercopy_abort()!
|
|gcc generated __builtin_trap
|Path: /bin/busybox
|CPU: 0 PID: 84 Comm: init Not tainted 5.4.22
|
|[ECR ]: 0x00090005 => gcc generated __builtin_trap
|[EFA ]: 0x9024fcaa
|[BLINK ]: usercopy_abort+0x8a/0x8c
|[ERET ]: memfd_fcntl+0x0/0x470
|[STAT32]: 0x80080802 : IE K
|...
|...
|Stack Trace:
| memfd_fcntl+0x0/0x470
| usercopy_abort+0x8a/0x8c
| __check_object_size+0x10e/0x138
| copy_strings+0x1f4/0x38c
| __do_execve_file+0x352/0x848
| EV_Trap+0xcc/0xd0

The issue is triggered by an allocation in "init reclaimed" region.
ARC _stext emcompasses the init region (for historical reasons we wanted
the init.text to be under .text as well). This however trips up
__check_object_size()->check_kernel_text_object() which treats this as
object bleeding into kernel text.

Fix that by rezoning _stext to start from regular kernel .text and leave
out .init altogether.

Fixes: https://github.com/foss-for-synopsys-dwc-arc-processors/linux/issues/15
Reported-by: Evgeniy Didin <didin@synopsys.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>