add rust_eh_personality to #[no_std] alloc tests

 ### Changes

    This commit addresses some test failures that now occur in the
    following two tests:

        * no_std-alloc-error-handler-custom.rs
        * no_std-alloc-error-handler-default.rs

    Each test now defines a `rust_eh_personality` extern function, in
    the same manner as shown in the "Writing an executable without
    stdlib" section of the `lang_items` documentation here:
    https://doc.rust-lang.org/unstable-book/language-features/lang-items.html#writing-an-executable-without-stdlib

    Without this change, these tests would fail to compile due to a
    linking error explaining that there was an "undefined reference
    to `rust_eh_personality'."

Author: katelyn a. martin

src/test/ui/allocator/no_std-alloc-error-handler-custom.rs

@@ -7,7 +7,7 @@
 // compile-flags:-C panic=abort
 // aux-build:helper.rs
 
-#![feature(start, rustc_private, new_uninit, panic_info_message)]
+#![feature(start, rustc_private, new_uninit, panic_info_message, lang_items)]
 #![feature(alloc_error_handler)]
 #![no_std]
 
@@ -84,6 +84,13 @@ fn panic(panic_info: &core::panic::PanicInfo) -> ! {
     }
 }
 
+// Because we are compiling this code with `-C panic=abort`, this wouldn't normally be needed.
+// However, `core` and `alloc` are both compiled with `-C panic=unwind`, which means that functions
+// in these libaries will refer to `rust_eh_personality` if LLVM can not *prove* the contents won't
+// unwind. So, for this test case we will define the symbol.
+#[lang = "eh_personality"]
+extern fn rust_eh_personality() {}
+
 #[derive(Debug)]
 struct Page([[u64; 32]; 16]);
 

src/test/ui/allocator/no_std-alloc-error-handler-default.rs

@@ -8,7 +8,7 @@
 // aux-build:helper.rs
 // gate-test-default_alloc_error_handler
 
-#![feature(start, rustc_private, new_uninit, panic_info_message)]
+#![feature(start, rustc_private, new_uninit, panic_info_message, lang_items)]
 #![feature(default_alloc_error_handler)]
 #![no_std]
 
@@ -71,6 +71,13 @@ fn panic(panic_info: &core::panic::PanicInfo) -> ! {
     }
 }
 
+// Because we are compiling this code with `-C panic=abort`, this wouldn't normally be needed.
+// However, `core` and `alloc` are both compiled with `-C panic=unwind`, which means that functions
+// in these libaries will refer to `rust_eh_personality` if LLVM can not *prove* the contents won't
+// unwind. So, for this test case we will define the symbol.
+#[lang = "eh_personality"]
+extern fn rust_eh_personality() {}
+
 #[derive(Debug)]
 struct Page([[u64; 32]; 16]);