boot: initialize .bss

Previously, the boot code assumed (by omission) that memory is initialized
to zero.  Apparently that is not the case, and this caused memory to become
corrupted.

Initialize .bss to zero to fix.

arch/x64/boot.s

@@ -45,7 +45,7 @@ tr:	.word     tss_desc - gdt
 
 .globl start32
 start32:
-    mov %eax, elf_header
+    mov %eax, %ebp
     lgdt gdt_desc
     mov $0x10, %eax
     mov %eax, %ds
@@ -74,5 +74,11 @@ start64:
     movb %dl, tss_desc + 4
     movb %dh, tss_desc + 7
     ltr tr
+    lea .bss, %rdi
+    lea .edata, %rcx
+    sub %rdi, %rcx
+    xor %eax, %eax
+    rep stosb
+    mov %rbp, elf_header
     jmp main
 

arch/x64/loader.ld

@@ -18,6 +18,7 @@ SECTIONS
     .init_array : { *(.init_array) } :text
     _init_array_end = .;
     .bss : { *(.bss) } :text
+    .edata = .;
 }
 PHDRS {
 	text PT_LOAD FILEHDR PHDRS;