[build] Inhibit .eh_frame on newer gcc versions

[build] Inhibit .eh_frame on newer gcc versions

Using -fno-dwarf2-cfi-asm is not sufficient to prevent the .eh_frame
section from being generated on newer versions of gcc.  Add
-fno-exceptions -fno-unwind-tables -fno-asynchronous-unwind-tables;
this is sufficient to inhibit the .eh_frame section on gcc 4.7.1.

This does not affect the overall binary size, but does fix the numbers
reported by "size" for individual object files.

Signed-off-by: Michael Brown <mcb30@ipxe.org>