Rework time functions a bit

- mktime: Work unsigned as time_t's type implies (shifting Y2K38 bug to 2106)
- mktime: Add unit tests
- gmtime/localtime: factorize
- gmtime/localtime: Add unit tests
- mktime/gmtime/localtime: Size optimisation (-130 bytes wrt master)
- mktime: Speed optimisation (from 23M cycles on the unit test to 2M)

libsrc/common/gmtime.s

@@ -0,0 +1,20 @@
+;
+; Colin Leroy-Mira, 2024
+;
+; struct tm* __fastcall__ gmtime (const time_t* timep);
+;
+
+        .export         _gmtime
+        .import         __time_t_to_tm
+        .import         ldeaxi
+
+_gmtime:
+        cpx     #$00            ; Check for null pointer
+        bne     :+
+        cmp     #$00
+        beq     no_pointer
+:       jsr     ldeaxi          ; Load value from pointer
+        jmp     __time_t_to_tm  ; Convert it
+
+no_pointer:
+        rts                     ; A/X already set