RISC-V/libgcc: Use `-fasynchronous-unwind-tables' for LIB2_DIVMOD_FUNCS

RISC-V/libgcc: Use `-fasynchronous-unwind-tables' for LIB2_DIVMOD_FUNCS

Use `-fasynchronous-unwind-tables' rather than `-fexceptions
-fnon-call-exceptions' in LIB2_DIVMOD_FUNCS compilation flags so as to
provide unwind tables for the affected functions while not pulling the
unwinder proper, which is not required here.

Beyond saving program space it fixes a RISC-V glibc build error due to
unsatisfied `malloc' and `free' references from the unwinder causing
link errors with `ld.so' where libgcc has been built at -O0.

	libgcc/
	* config/riscv/t-elf (LIB2_DIVMOD_EXCEPTION_FLAGS): New
	variable.