48-bit bitwise not emits `__snot` instead of `__i48not`

Demonstration: https://discord.com/channels/432891584451706892/1184990251769397380/1305595574246182952

> Compiling c -std=gnu11 in `-Oz` for ez80:
> ```cpp
> int48_t foo(int48_t x) { return ~x; }
> ```
> ```avrasm
> public	_foo
> _foo:
> 	call	__frameset0
> 	ld	hl, (ix + 6)
> 	ld	de, (ix + 9)
> 	call	__snot
> 	pop	ix
> 	ret
> 
> 	extern	__snot
> 	extern	__frameset0
> ```