[rfc] Skip non-default multilib and libstdc++-v3 debug builds in bootstrap builds

[Matthias Klose]

The attached patch avoids building the non-default multilib and libstdc++-v3 
debug builds in bootstrap builds during bootstrap builds, resulting in some 
speedup for bootstrap builds for targets with multilibs enabled.  On 
x86_64-linux-gnu with 64/32/x32 multilibs enabled and the libstdc++ debug build 
enabled the bootstrap with -j8 takes 34:10 without, and with the patch 26:55, 
making the build ~20% faster (standard languages plus go enabled).  It's faster 
because it avoids the rather sequential libcc/libstdc++/libgomp configury during 
stage1 and stage2.  The gain by not building the libstdc++ debug library is not 
that measurable, the config is copied, there is no separate configure run.  Is 
relying on stage_current/stage_final the correct approach?  Should that be 
enabled using a configure option, or by default?

Matthias

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