[CORE] vmm_stdio: Improve arch_defterm_early_putc() usage

Currently, arch_defterm_early_putc() is optional and if arch implements
it then arch_config.h will define ARCH_HAS_DEFTERM_EARLY_PRINT.

There are lot other optional arch_xxx() APIs but generally core code
provides default weak implementation of these APIs and arch code
provides hard implementation of these APIs if it wants to implement.

For consistency with other optional arch_xxx() APIs we provide default
weak implemenations of arch_defterm_early_putc() in vmm_stdio and
remove ARCH_HAS_DEFTERM_EARLY_PRINT from arch_config.h.

Signed-off-by: Anup Patel <anup@brainfault.org>