Fix various typos in comments and printed messages

Fix various typos in comments and printed messages

This patch contains some spelling fixes (just in comments and C strings
passed to printf) as found by bot (https://github.com/ka7/misspell_fixer).

* SMakefile.amiga: Fix typo in comment ("don't").
* WCC_MAKEFILE (SYSTEM): Fix typo in comment ("uncomment").
* configure.ac [!GCC] (gc_cflags): Fix typo in comment ("actually").
* configure.ac (NO_CLOCK): Fix typo in comment ("targeting").
* doc/README.amiga: Fix typos in documentation ("there's", "compliant",
"documentation").
* extra/AmigaOS.c [GC_AMIGA_PRINTSTATS] (GC_amiga_free_all_mem): Fix
typos in printed messages ("succeeded", "succeeding").
* include/gc.h (GC_get_heap_size): Fix typo in comment ("below").
* tools/setjmp_t.c (main): Fix typo in printed message ("won't").