emacs: use -B flags for native compilation dependencies
emacs: use -B flags for native compilation dependencies

The -B flag to gcc (and libgccjit) allows us to specify where it can
find things it needs to correctly compile code (both programs and
libraries) without adjusting any environmental flags: So, no need to
wrap the program for a PATH entry containing binutils, and no need to
explicitly pass a linker path anymore.

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