[Patch] Don't declare all text sections at the start of the .s

[Rafael Espíndola]

A version with explicit section names and more comments is attached.

I think this addresses all concerns:

* I can bootstrap with debug info on X86_64.
* It can be used to build some objc to arm.
* If a non-whitelisted section is created after dwarf, we assert.

The white listed sections are the one that the assembler itself
creates at the end of the file.

OK?
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