Support multilib properties in bp2build

This combines properties among "multilib" and "arch" stanzas in selects
generated by bp2build.

With this fix, libc_gdtoa may be removed from the denylist.

This change also refactors a portion of arch.go, specifically bp2build's
arch mutator, adding a number of comments along the way, to hopefully
make this code clearer for future readers.

Test: mixed_libc.sh
Change-Id: If2beea672957cfb1af6760406ba507181ec38f77