[kernel] Hide kernel symbols by default

Using `-fvisibility=hidden` when building the kernel, and then
`--discard-all` when stripping it, we shave almost 100KiB off of the
resulting ELF file.

Also dropped some unused symbols from the linker script, and rearranged
the sections so that the file is able to be mapped directly into memory
instead of having each section copied.

src/kernel/boot.s

@@ -2,7 +2,7 @@ MAGIC	equ  'j6KERNEL'	; jsix kernel header magic number
 
 section .header
 align 8
-global _kernel_header
+global _kernel_header: data hidden
 _kernel_header:
 	dq MAGIC            ; Kernel header magic
 	dw 32               ; Kernel header length
@@ -16,8 +16,8 @@ _kernel_header:
 
 section .text
 align 16
-global _kernel_start:function (_kernel_start.end - _kernel_start)
-global _kernel_start.real
+global _kernel_start: function hidden (_kernel_start.end - _kernel_start)
+global _kernel_start.real: function hidden
 _kernel_start:
 	push rbp     ; Never executed, fake function prelude
 	mov rbp, rsp ; to calm down gdb
@@ -36,18 +36,18 @@ _kernel_start:
     ; the scheduler to take over
 .end:
 
-global bsp_idle:function (bsp_idle.end - bsp_idle)
+global bsp_idle: function hidden (bsp_idle.end - bsp_idle)
 bsp_idle:
 	hlt
 	jmp bsp_idle
 .end:
 
-global interrupts_enable
+global interrupts_enable: function hidden
 interrupts_enable:
 	sti
 	ret
 
-global interrupts_disable
+global interrupts_disable: function hidden
 interrupts_disable:
 	cli
 	ret
@@ -57,5 +57,5 @@ align 0x100
 idle_stack_begin:
 	resb 0x4000 ; 16KiB stack space
 
-global idle_stack_end
+global idle_stack_end: data hidden
 idle_stack_end: