unicore32: convert get_sp_from_cpustate from macro to inline

unicore32: convert get_sp_from_cpustate from macro to inline

All other architectures define get_sp_from_cpustate as an inline function,
only unicore32 uses a #define.  With this, some usages are impossible, for
example, enabling sigaltstack in linux-user/syscall.c results in

linux-user/syscall.c: In function ‘do_syscall’:
linux-user/syscall.c:8299:39: error: dereferencing ‘void *’ pointer [-Werror]
  get_sp_from_cpustate(arg1, arg2, get_sp_from_cpustate((CPUArchState *)cpu_env));
                                       ^
linux-user/syscall.c:8299:39: error: request for member ‘regs’ in something not a structure or union

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>