build system: stop .eh_frame generation

From Rich Felker:

By default, modern GCC generates DWARF2 debug/unwind tables in the
.eh_frame section of the object files/binaries. This adds significant
bloat (as much as 15%) to the size of the busybox binary, including
the portion mapped/loaded into memory at runtime (possibly a big issue
for NOMMU targets), and the section is not strippable with the strip
command due to being part of the loaded program text.

I've since done some further checking - both testing and asking the
GCC developers about it - and it seems the solution is to add to the
CFLAGS -fno-unwind-tables and -fno-asynchronous-unwind-tables. If
debugging is disabled, this will prevent GCC from outputting DWARF2
tables entirely. But since busybox builds with -g by default, the
interesting case is what happens then. I originally thought these
options would break debugging, but they don't; instead, they tell GCC
to output the DWARF2 tables in the .debug_frame section instead of
the newish .eh_frame section (used for exception handling). With these
options added, busybox_unstripped is still fully debuggable, and the
final busybox binary loses the 15% bloat factor from the DWARF2
tables.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

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