Re-port alloc.c to Solaris sparc and simplify

Re-port alloc.c to Solaris sparc and simplify

alloc.c had bitrotted a bit, and used an undefined symbol
stack_base when Emacs was built on Solaris sparc, leading to
compilation failures.  Also, code related to __builtin_unwind_init
was unnecessarily duplicated.  Fix the bitrot and remove some
duplication.
* src/alloc.c: Remove uses of GC_SAVE_REGISTERS_ON_STACK, since it
is never defined.
(test_setjmp) [!HAVE___BUILTIN_UNWIND_INIT && GC_SETJMP_WORKS]:
Define a no-op dummy, to simplify use.
(test_setjmp) [!GC_SETJMP_WORKS]: Test setjmp_tested_p here rather
than in the caller, to simplify use.
(stacktop_sentry): New type.
(__builtin_unwind_init) [!HAVE___BUILTIN_UNWIND_INIT]: New macro.
(SET_STACK_TOP_ADDRESS): New macro, containing code that was duplicated.
(flush_stack_call_func, Fgarbage_collect): Use it.
(init_alloc): Omit unnecessary initialization.
After dumping, Emacs need not re-test setjmp.