Remove morestack support

This commit removes all morestack support from the compiler which entails:

* Segmented stacks are no longer emitted in codegen.
* We no longer build or distribute libmorestack.a
* The `stack_exhausted` lang item is no longer required

The only current use of the segmented stack support in LLVM is to detect stack
overflow. This is no longer really required, however, because we already have
guard pages for all threads and registered signal handlers watching for a
segfault on those pages (to print out a stack overflow message). Additionally,
major platforms (aka Windows) already don't use morestack.

This means that Rust is by default less likely to catch stack overflows because
if a function takes up more than one page of stack space it won't hit the guard
page. This is what the purpose of morestack was (to catch this case), but it's
better served with stack probes which have more cross platform support and no
runtime support necessary. Until LLVM supports this for all platform it looks
like morestack isn't really buying us much.

cc #16012 (still need stack probes)
Closes #26458 (a drive-by fix to help diagnostics on stack overflow)

src/libstd/sys/windows/c.rs

@@ -78,6 +78,10 @@ pub const PROGRESS_QUIET: libc::DWORD = 3;
 pub const TOKEN_ADJUST_PRIVILEGES: libc::DWORD = 0x0020;
 pub const SE_PRIVILEGE_ENABLED: libc::DWORD = 2;
 
+pub const EXCEPTION_CONTINUE_SEARCH: LONG = 0;
+pub const EXCEPTION_MAXIMUM_PARAMETERS: usize = 15;
+pub const EXCEPTION_STACK_OVERFLOW: DWORD = 0xc00000fd;
+
 #[repr(C)]
 #[cfg(target_arch = "x86")]
 pub struct WSADATA {
@@ -327,6 +331,24 @@ pub struct REPARSE_MOUNTPOINT_DATA_BUFFER {
     pub ReparseTarget: libc::WCHAR,
 }
 
+#[repr(C)]
+pub struct EXCEPTION_RECORD {
+    pub ExceptionCode: DWORD,
+    pub ExceptionFlags: DWORD,
+    pub ExceptionRecord: *mut EXCEPTION_RECORD,
+    pub ExceptionAddress: LPVOID,
+    pub NumberParameters: DWORD,
+    pub ExceptionInformation: [LPVOID; EXCEPTION_MAXIMUM_PARAMETERS]
+}
+
+#[repr(C)]
+pub struct EXCEPTION_POINTERS {
+    pub ExceptionRecord: *mut EXCEPTION_RECORD,
+    pub ContextRecord: LPVOID
+}
+
+pub type PVECTORED_EXCEPTION_HANDLER = extern "system"
+        fn(ExceptionInfo: *mut EXCEPTION_POINTERS) -> LONG;
 
 #[link(name = "ws2_32")]
 #[link(name = "userenv")]
@@ -487,6 +509,9 @@ extern "system" {
                                  BufferLength: libc::DWORD,
                                  PreviousState: PTOKEN_PRIVILEGES,
                                  ReturnLength: *mut libc::DWORD) -> libc::BOOL;
+    pub fn AddVectoredExceptionHandler(FirstHandler: ULONG,
+                                       VectoredHandler: PVECTORED_EXCEPTION_HANDLER)
+                                       -> LPVOID;
 }
 
 // Functions that aren't available on Windows XP, but we still use them and just