KASLR: add compile-time option to disable randomization

A kernel built with KASLR disabled is more debug-friendly.
Edit the gdb initialization files so that .gdbinit contains only
commands common to all architectures, while the .gdbinit-* files
contain architecture-specific commands (where the kernel symbol
offset is set to the value used when KASLR is disabled) and are
meant to be used via the `-ix` gdb command line option.

Author: francescolavra

.gdbinit

@@ -1,10 +1,5 @@
-set architecture i386:x86-64
-
 macro define offsetof(t, f) (size_t)&((t *)0)->f
 macro define container_of(p, t, f) (t *)((void *)p - offsetof(t, f))
 
 source tools/nanos_gdb.py
-display/i $pc
-symbol-file ./output/platform/pc/bin/kernel.elf
-target remote :1234
 

.gdbinit-aarch64

@@ -1,5 +1,4 @@
 set architecture aarch64
-display/i $pc
+file ./output/platform/virt/bin/kernel.elf -o 0xffffffff3fc00000
 target remote :1234
-symbol-file ./output/platform/virt/bin/kernel.elf
 

.gdbinit-riscv64

@@ -1,5 +1,4 @@
 set architecture riscv:rv64
-display/i $pc
+file ./output/platform/riscv-virt/bin/kernel.elf -o 0xfffffffeffe00000
 target remote :1234
-symbol-file ./output/platform/riscv-virt/bin/kernel.elf
 

.gdbinit-x86-64

@@ -0,0 +1,4 @@
+set architecture i386:x86-64
+file ./output/platform/pc/bin/kernel.elf -o 0xffffffff7fe00000
+target remote :1234
+

src/kernel/init.c

@@ -63,8 +63,12 @@ BSS_RO_AFTER_INIT static vector shutdown_completions;
 void kaslr(void)
 {
     extern u8 START, text_end, READONLY_END, bss_start, END;
+#ifndef NO_KASLR
     u64 ksize = pad(&END - &START, PAGESIZE);
     u64 random_offset = random_early_u64() % (KERNEL_LIMIT - KERNEL_BASE - ksize);
+#else
+    u64 random_offset = 0;
+#endif
     u64 kbase = KERNEL_BASE + (random_offset & ~PAGEMASK);
     u64 phys_offset = kernel_phys_offset;
     u64 kern_offset = kbase - u64_from_pointer(&START) + phys_offset;