Added clock_settime() for some CBMs.

The CIA TOD only stores the time but not the date. Therefore the date set by clock_settime() ist just stored inside the C library for retrieval via clock_gettime().

The "very special" handling of 12AM/PM is based on https://groups.google.com/d/msg/comp.sys.cbm/ysVYSX4AMbc/vHrXCWEhCOUJ saying:

==========

24hr: Wr => Rd => Nx
--------------------
  0 : 92 => 12 => 01   <= Switch from 00 to 01 (24-hour notation)
  1 : 01 => 01 => 02
  2 : 02 => 02 => 03
 11 : 11 => 11 => 92
 12 : 12 => 92 => 81   <= Switch from 12 to 13 (24-hour notation)
 13 : 81 => 81 => 82
 14 : 82 => 82 => 83
 23 : 91 => 91 => 12

1. column ("24hr"): hour to be tested (decimal)
2. column ("Wr"):   hour written to TOD register (BCD)
3. column ("Rd"):   hour read from TOD register (BCD) immediately after writing the value in column 2 to see the conversion between AM/PM, if any
4. column ("Nx"):   next hour (BCD) after the hour switch

==========

Thanks Paul!

libsrc/cbm510/tmcommon.s

@@ -0,0 +1,41 @@
+;
+; Oliver Schmidt, 16.8.2018
+;
+; Common stuff for the clock routines
+;
+
+        .include        "cbm510.inc"
+
+        .export         TM, load_tenth
+
+        .importzp       sreg
+
+
+;----------------------------------------------------------------------------
+.code
+
+.proc   load_tenth
+
+        lda     #<(100 * 1000 * 1000 / $10000)
+        ldx     #>(100 * 1000 * 1000 / $10000)
+        sta     sreg
+        stx     sreg+1
+        lda     #<(100 * 1000 * 1000)
+        ldx     #>(100 * 1000 * 1000)
+        rts
+
+.endproc
+
+;----------------------------------------------------------------------------
+; TM struct with date set to 1970-01-01
+.data
+
+TM:     .word           0       ; tm_sec
+        .word           0       ; tm_min
+        .word           0       ; tm_hour
+        .word           1       ; tm_mday
+        .word           0       ; tm_mon
+        .word           70      ; tm_year
+        .word           0       ; tm_wday
+        .word           0       ; tm_yday
+        .word           0       ; tm_isdst