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/tests/objects.mk

@@ -24,3 +24,8 @@ libsbi-objs-$(CONFIG_SBIUNIT) += tests/sbi_bitops_test.o
 
 carray-sbi_unit_tests-$(CONFIG_SBIUNIT) += string_test_suite
 libsbi-objs-$(CONFIG_SBIUNIT) += tests/sbi_string_test.o
+
+ifeq ($(UBSAN),y)
+carray-sbi_unit_tests-$(CONFIG_SBIUNIT) += ubsan_test_suite
+libsbi-objs-$(CONFIG_SBIUNIT) += tests/sbi_ubsan_test.o
+endif