arch/arm64/armv8: Add exception output without printk

In case printk does not work the current exception handler will print a
simple "!" to notify the developer that coreboot is actually there but
something went wrong.

The "!" can be quite confusing when it actually happens that printk does
not work. Since "!" doesn't really say much (if you don't know the
exception arm64 code) the developer (like me) can easily assume that
something went wrong while configuring clocks or baud rate of UART,
since the output seemingly does not seem to make sense.

This adds a little bit more output to assure the developer that what was
printed was actually intended to be printed. Therefore it prints
"EXCEPT" which assures the developer that this was intended output.
It also adds a comment above so that developer can more easily grep
for this message.

It has intentionally not been written as:
```
const char *msg = "\r\n!EXCPT!";
while (*msg)
  __uart_tx_byte(*msg++);
```
because in this case the compiler will generate code that will place
`msg` somewhere in bootblock and the code will try to access this using
a memory address. In rare cases (if you link bootblock at the wrong
address) this memory address can be wrong and coreboot will not print
the message. Using individual calls to `__uart_tx_byte` ensures that the
compiler will generate code which directly puts the character bytes into
the argument register without referencing a variable in bootblock.

Signed-off-by: Maximilian Brune <maximilian.brune@9elements.com>
Change-Id: I2f858730469fff3cae120fd7c32fec53b3d309ca
---
M src/arch/arm64/armv8/exception.c
1 file changed, 12 insertions(+), 1 deletion(-)