porters handbook: add wordsize information to architecture section

[linimon]

Summary

This builds on D19233 by adding new text about how to deal with e.g. different wordsizes on differernt architectures.

[mat]

Maybe also add something about {C,CXX,LD}FLAGS_${ARCH} existence.

Also, please add an blank line after the <title> lines.

chapter.xml

560–562

These should maybe be examples of how to use ONLY_FOR_ARCH and NOT_FOR_ARCH instead.

574–575

Maybe make -C /usr/src targets should probably be pointed to first, before asking people to go read a file.

[linimon]

Complete rewrite of patch. Incorporate feedback. Move general information higher. Use better shorthand for 32/64 bit test.

[linimon]

I also wanted to punt the discussion of *FLAGS_<arch> until another patch, so I changed the text of the 64/32 bit examples.

[linimon]

Minor cleanup.

[mat]

en_US.ISO8859-1/books/porters-handbook/porting-dads/chapter.xml

612 ↗

(On Diff #59576)

Unless I am mistaken, this is not valid make(1) code. It should probably be :M*64*. Same below.

[linimon]

Fix broken sample code from suggestion.