Add option to send additional build args to ca65

[zohassadar]

No description provided.

[zohassadar]

Tested with the following added to constants.asm (it's built twice so it everything outputs twice) to validate multiple flags, stderr/stdout behavior and exiting on error.

.out "this is stdout material"

.ifdef WARNME
.warning "you've been warned"
.endif

.ifdef FAIL
.error "failed"
.endif