[M1] kernel_main: print "hello" over VGA/serial

[mitsuakki]

First C entry point, called after the jump to long mode. Zero .bss before calling (loop from linker-defined __bss_start to __bss_end). Proves the entire chain: GDT, paging, long mode transition, and the ASM-to-C calling boundary all work together.

Resources:

• System V AMD64 ABI — calling convention for the asm/C boundary in long mode