Toolchain/GCC+LibC: Stop linking against crt0_shared

This makes GCC and Clang equal on that side and thus allows us to get
rid of most GCC-only branches in LibC/CMakeLists.txt.

Toolchain/Patches/gcc/0001-Add-a-gcc-driver-for-SerenityOS.patch

@@ -100,7 +100,7 @@ index 0000000000000000000000000000000000000000..53a4b8e93b74b4808a4bfed91c4d5558
 +#define PTRDIFF_TYPE    (TARGET_64BIT ? "long int" : "int")
 diff --git a/gcc/config/serenity.h b/gcc/config/serenity.h
 new file mode 100644
-index 0000000000000000000000000000000000000000..effbe4bfb4c35db4785491e1628e6af9c0c87bf0
+index 0000000000000000000000000000000000000000..5b6c764c9bbf68fcf307aa67a19f6fc4305f74ef
 --- /dev/null
 +++ b/gcc/config/serenity.h
 @@ -0,0 +1,50 @@
@@ -120,7 +120,7 @@ index 0000000000000000000000000000000000000000..effbe4bfb4c35db4785491e1628e6af9
 +/* Files that are linked before user code.
 +   The %s tells GCC to look for these files in the library directory. */
 +#undef STARTFILE_SPEC
-+#define STARTFILE_SPEC "%{!shared:crt0.o%s} %{shared: %{!fbuilding-libgcc:crt0_shared.o%s}} %{shared|static-pie|!no-pie:crtbeginS.o%s; :crtbegin.o%s}"
++#define STARTFILE_SPEC "%{!shared:crt0.o%s} %{shared|static-pie|!no-pie:crtbeginS.o%s; :crtbegin.o%s}"
 +
 +/* Files that are linked after user code. */
 +#undef ENDFILE_SPEC

Userland/Libraries/LibC/CMakeLists.txt

@@ -103,17 +103,6 @@ add_custom_command(
     COMMAND "${CMAKE_COMMAND}" -E copy $<TARGET_OBJECTS:crt0> ${CMAKE_INSTALL_PREFIX}/usr/lib/crt0.o
 )
 
-# FIXME: These files are never needed (and Clang driver never references them) but GCC might need
-#        some convincing that this is the case.
-if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
-    add_library(crt0_shared STATIC crt0_shared.cpp)
-    target_link_libraries(crt0_shared PRIVATE NoCoverage)
-    add_custom_command(
-        TARGET crt0_shared POST_BUILD
-        COMMAND "${CMAKE_COMMAND}" -E copy $<TARGET_OBJECTS:crt0_shared> ${CMAKE_INSTALL_PREFIX}/usr/lib/crt0_shared.o
-    )
-endif()
-
 # ===== LibC =====
 # Prevent GCC from removing null checks by marking the `FILE*` argument non-null
 set_source_files_properties(stdio.cpp PROPERTIES COMPILE_FLAGS "-fno-builtin-fputc -fno-builtin-fputs -fno-builtin-fwrite")
@@ -127,9 +116,6 @@ endif()
 
 serenity_libc(LibC c)
 add_dependencies(LibC crt0 LibUBSanitizer)
-if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
-    add_dependencies(LibC crt0_shared)
-endif()
 target_link_libraries(LibC PRIVATE LibSystem LibTimeZone)
 target_link_options(LibC PRIVATE -nolibc)
 

Userland/Libraries/LibC/crt0_shared.cpp

@@ -1,15 +0,0 @@
-/*
- * Copyright (c) 2020, the SerenityOS developers.
- *
- * SPDX-License-Identifier: BSD-2-Clause
- */
-
-#include <AK/Types.h>
-#include <assert.h>
-#include <stdio.h>
-#include <sys/internals.h>
-#include <unistd.h>
-
-#ifndef _DYNAMIC_LOADER
-void* __dso_handle __attribute__((__weak__));
-#endif