Printing double semicolons clashes with double underscores

[sarranz]

Jean-Christophe noticed that this program

export fn ble::e() -> reg u64 {
    reg u64 x = 0;
    return x;
}

export fn ble__e() -> reg u64 {
    reg u64 x = 1;
    return x;
}

generates

_ble__e:
ble__e:
	movq	$1, %rax
	ret
_ble__e:
ble__e:
	movq	$0, %rax
	ret

which is not good.
The compiler does check that we don't have a namespace ble with a function e, so we probably want to extend that?.