elf, uapi: Add definitions for VER_FLG_BASE and VER_FLG_WEAK

The definitions are used by tools/testing/selftests/vDSO/parse_vdso.c.

To be able to build the vDSO selftests without a libc dependency,
add the definitions to the kernels own UAPI headers.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kees Cook <kees@kernel.org>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://docs.oracle.com/cd/E19683-01/816-1386/chapter6-80869/index.html
Link: https://lore.kernel.org/all/20250226-parse_vdso-nolibc-v2-4-28e14e031ed8@linutronix.de