bsd-user/target_os_elf: If ELF_HWCAP2 is defined, publish it

bsd-user/target_os_elf: If ELF_HWCAP2 is defined, publish it

Some architectures publish AT_HWCAP2 as well as AT_HWCAP. Those
architectures will define ELF_HWCAP2 in their target_arch_elf.h files
for the value for this process. If it is defined, then publish it.

Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Kyle Evans <kevans@FreeBSD.org>