[patch, build] Restore bootstrap in building libcc1 on darwin

[FX]

> The default BOOT_CFLAGS are: -O2 -g -mdynamic-no-pic
> the libiberty pic build appends: -fno-common (and not even -fPIC) [NB -fPIC _won't_ override -mdynamic-no-pic, so that's not a simple way out]
> This means that the PIC library is being built with non-pic relocs.

config/mh-darwin says that -mdynamic-no-pic is there because it “speeds compiles by 3-5%”. I don’t think we care about speed when the bootstrap fails, so can we remove it altogether?

FX


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