Kernel+Toolchain: Use .init_array section for global constructors

Before this change, our dynamic linker's global constructor handler
relied on the GNU linker implicitly including the content of `.ctors`
section inside `.init_array`. The mold linker does not do this, so
global constructors would fail to be called in the mold-built userland.

There is no point in sticking to `.ctors`, as most other systems already
use the superior `.init_array` scheme. This commit changes the kernel
linker script to not discard this new section, and enables it by default
in our toolchain.

Kernel/linker.ld

@@ -59,10 +59,12 @@ SECTIONS
     {
         start_heap_ctors = .;
         *libkernel_heap.a:*(.ctors)
+        *libkernel_heap.a:*(.init_array)
         end_heap_ctors = .;
 
         start_ctors = .;
         *(.ctors)
+        *(.init_array)
         end_ctors = .;
 
         *(.rodata*)