linux-user/alpha: Add define for NR_shmat to enable shmat syscall

linux-user/alpha: Add define for NR_shmat to enable shmat syscall

For historical reasons, the define for the shmat() syscall on Alpha is
NR_osf_shmat; however it has the same semantics as this syscall does
on all other architectures, so define TARGET_NR_shmat as well so that
QEMU's code for the syscall is enabled.

This patch brings our behaviour on the LTP shmat tests into line
with that for ARM (still not a perfect pass rate but not "this syscall
is completely broken" as we had before).

(Problem detected via a clang warning that the do_shmat() function
was unused on Alpha.)

Cc: Richard Henderson <rth@twiddle.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>