Fix local labels not working after a .msg directive

[StraDaMa]

Currently if you try to use a local label after a .msg directive like this

.gba
.create "test.bin", 0x00000000
start:
cmp r1,0x64
bne @@end
.msg "test"
@@end:
mov r15,r14
.close

it will say that @@end is an undefined label. This change has it generate a local label so it doesn't cause the local labels after it to become invalid.