[libunwind] Move the alias definition of unw_getcontext to within !defined(__USING_SJLJ_EXCEPTIONS__)

[mstorsjo]

Summary

For builds with SJLJ, there is no __unw_getcontext symbol. On Windows, the weak alias macro also expands to a dllexport directive, which fails if the symbol doesn't exist.

Apparently I had forgotten to test actual builds with SJLJ, sorry for that omission.

src/UnwindRegistersSave.S

DEFINE_LIBUNWIND_FUNCTION(__unw_getcontext)
   std %l6, [%o0 +  88]
   std %i0, [%o0 +  96]
   std %i2, [%o0 + 104]
   std %i4, [%o0 + 112]
   std %i6, [%o0 + 120]
   jmp %o7
    clr %o0                   // return UNW_ESUCCESS
 #endif
-#endif /* !defined(__USING_SJLJ_EXCEPTIONS__) */
 
   WEAK_ALIAS(__unw_getcontext, unw_getcontext)
 
+#endif /* !defined(__USING_SJLJ_EXCEPTIONS__) */
+
 NO_EXEC_STACK_DIRECTIVE