add arch.7 man page with architecture-specific details

[emaste]

Summary

Based on https://wiki.freebsd.org/EdMaste/ArchitectureSpecifics

Needs more work; posting for now to get feedback on correct markup

[emaste]

share/man/man7/arch.7

112

One missing part from the wiki is footnotes on some of these:

double precision[1]

[1] Prior to FreeBSD 11, armv6 is built with the softfp ABI by default, it uses hard float, but passed floating-point arguments as soft float, i.e. in integer registers.

[wblock]

share/man/man7/arch.7

42

This "size" should be capitalized. The later ones are part of reserved words.

126

Built-in and predefined really mean the same thing, so this is redundant. Predefined is probably the better choice for a single one.

131

s/the following/this/

[emaste]

• connect to Makefile
• incorporate improvements from @wblock

[jhb]

It should be trivial to merge new manpages into 11 btw, so I think the history should optimistically reference 11.0. :)

Another interesting category (which can be added later) is what ELF header constants are used for different platforms. (I had to figure this out for the libkvm changes. For example, mipsn32 and mipso32 share the EM_xxx and use a flag to differentiate)

[andrew]

The ARM bits look correct.

share/man/man7/arch.7

112

That would only be an issue if we merged this into 10.