mescc: Produce object files in hex2 format, remove hex3.
* GNUmakefile (HEX2_FLAGS): New variable.
* make/mescc-guile.make ($(OUT)/$(TARGET)): Use HEX2 for linking.
* make/mescc-mes.make ($(OUT)/$(TARGET)): Likewise.
* guile/mescc.scm (main): Remove hex3 support.
* scripts/mescc.mes (main): Likewise.
* stage0/elf32-0header.hex2: New file, merging of elf32.hex and elf32-header.hex2.
* stage0/elf-0footer.hex2: New file.
* stage0/elf32-header.hex2: Rename from elf32-header-exit-42.hex2,
repurpose as generic debug heder.
* stage0/elf32-footer-single-main.hex2: Rename from elf32-footer-exit-42.hex2,
repurpose as generic debug footer for single-main source.
* stage0/exit-42.c: New file.
* stage0/stage0.make ($(OUT)/0exit-42): Test 0header, 0footer.
($(OUT)/exit-42.guile): Test generic debug header, footer.
* stage0/elf32.hex2: Remove.
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