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/gettime.s

@@ -12,6 +12,7 @@
 
         .import         pushax, pusheax, tosmul0ax, steaxspidx, incsp1
         .import         sys_bank, restore_bank
+        .import         TM, load_tenth
         .importzp       sreg, tmp1, tmp2
 
 
@@ -54,12 +55,7 @@
         ldy     #timespec::tv_sec
         jsr     steaxspidx      ; Pops address pushed by 2. pushax
 
-        lda     #<(100 * 1000 * 1000 / $10000)
-        ldx     #>(100 * 1000 * 1000 / $10000)
-        sta     sreg
-        stx     sreg+1
-        lda     #<(100 * 1000 * 1000)
-        ldx     #>(100 * 1000 * 1000)
+        jsr     load_tenth
         jsr     pusheax
         ldy     #CIA::TOD10
         lda     (cia2),y
@@ -96,17 +92,3 @@
         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