CONFIG_GENERIC_SIGALTSTACK build breakage with asm-generic/syscalls.h

Saner transition plan for GENERIC_SIGALTSTACK conversion - instead of
adding #define sys_sigaltstack sys_sigaltstack in asm/syscalls.h of
architecture if it's pulls asm-generic/syscalls.h, only to have those
defines removed once all architectures are converted, make the
declaration in said asm-generic/syscalls.h conditional on the lack
of GENERIC_SIGALTSTACK.  Less messy in intermediate stages that way...

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: james.hogan@imgtec.com
Cc: arnd@arndb.de
Cc: torvalds@linux-foundation.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-arch@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>