lib: sbi: add UBSan support

UBSan (Undefined Behavior Sanitizer) is a tool implemented using
compiler instrumentation at runtime that allows checking for
statements whose output is not deterministic or defined by the C
standard. Compiling and running OpenSBI with UBSan instrumentation
will print a message in the console if any sentence performs such
an action.

Support involves two main components:
1. The UBSan implementation hooks (derived from NetBSD),
   used by the compiler to handle the check output.
2. A test suite integrated with the SBI unit test framework to
   verify correct operation at runtime.

Usage:

  make UBSAN=y PLATFORM=generic ...

The test suite is built when both UBSAN=y and CONFIG_SBIUNIT=y are
enabled.

When UBSan is enabled, FW_PAYLOAD_OFFSET may need to be increased
due to the size increase added by the instrumentation. A
value of 0x400000 has been tested.

UBSan adds runtime overhead and is intended for development builds
only, not for production.

Note: This patch marks __stack_chk_guard in sbi_init.c as a weak
symbol to prevent multiple definition errors at compile time with
UBSan instrumentation enabled. This resolves the conflict
between the .globl definitions in sbi_init.c and test_head.S.

Signed-off-by: Marcos Oduardo <marcos.oduardo@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260515163321.2038366-1-marcos.oduardo@gmail.com
Signed-off-by: Anup Patel <anup@brainfault.org>

lib/sbi/objects.mk

@@ -100,6 +100,9 @@ libsbi-objs-y += sbi_tlb.o
 libsbi-objs-y += sbi_trap.o
 libsbi-objs-y += sbi_trap_ldst.o
 libsbi-objs-y += sbi_trap_v_ldst.o
+ifeq ($(UBSAN), y)
+libsbi-objs-y += sbi_ubsan.o
+endif
 libsbi-objs-y += sbi_unpriv.o
 libsbi-objs-y += sbi_expected_trap.o
 libsbi-objs-y += sbi_cppc.o