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/x86_64/crti.S

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2021, Daniel Bertalan <dani@danielbertalan.dev>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+.section .init, "ax", @progbits
+.align 4
+.global _init
+.type _init, @function
+_init:
+    pushq %rbp
+    movq %rsp, %rbp
+    andq $-16, %rsp
+
+.section .fini, "ax", @progbits
+.align 4
+.global _fini
+.type _fini, @function
+_fini:
+    pushq %rbp
+    movq %rsp, %rbp
+    andq $-16, %rsp