LibC: Use our implementation of crti.o and crtn.o

We have had these for quite a while, but we didn't compile them, and
used GCC's version instead. Clang does not come with these, so we have
to provide our own implementation.

Our implementation follows what `musl` and `FreeBSD` do, so this should
work fine, even if documentation can hardly be found for them.

Userland/Libraries/LibC/arch/i386/crti.S

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+.section .init, "ax", @progbits
+.align 4
+.global _init
+.type _init, @function
+_init:
+    pushl %ebp
+    movl %esp, %ebp
+    andl $-16, %esp
+
+.section .fini, "ax", @progbits
+.align 4
+.global _fini
+.type _fini, @function
+_fini:
+    pushl %ebp
+    movl %esp, %ebp
+    andl $-16, %esp

Userland/Libraries/LibC/arch/i386/crtn.S

@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+.section .init, "ax", @progbits
+    movl %ebp, %esp
+    popl %ebp
+    retl
+
+.section .fini, "ax", @progbits
+    movl %ebp, %esp
+    popl %ebp
+    retl