allow libc itself to be built with stack protector enabled

this was already essentially possible as a result of the previous
commits changing the dynamic linker/thread pointer bootstrap process.
this commit mainly adds build system infrastructure:

configure no longer attempts to disable stack protector. instead it
simply determines how so the makefile can disable stack protector for
a few translation units used during early startup.

stack protector is also disabled for memcpy and memset since compilers
(incorrectly) generate calls to them on some archs to implement
struct initialization and assignment, and such calls may creep into
early initialization.

no explicit attempt to enable stack protector is made by configure at
this time; any stack protector option supported by the compiler can be
passed to configure in CFLAGS, and if the compiler uses stack
protector by default, this default is respected.

src/env/__stack_chk_fail.c

@@ -16,3 +16,13 @@ void __stack_chk_fail(void)
 {
 	a_crash();
 }
+
+#ifdef SHARED
+
+__attribute__((__visibility__("hidden")))
+void __stack_chk_fail_local(void)
+{
+	a_crash();
+}
+
+#endif