a BOOTSTRAP_ELF Makefile variable that, when set (e.g. in the environment),
builds a targeted-to-ELF toolchain without requiring OBJECT_FMT to be set
to "ELF" (which affects how e.g. libraries are built).
This basically means you don't need fake crtbegin.o and crtend.o files to
be present in /usr/lib while bootstrapping the toolchain.
6d2943920a