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...