libgcc: Use may_alias attribute in bitint handlers

As discussed on IRC, the following patch uses may_alias attribute, so that
on targets like aarch64 where abi_limb_mode != limb_mode the library
accesses the limbs (half limbs of the ABI) in the arrays with conservative
alias set.

2024-01-12  Jakub Jelinek  <jakub@redhat.com>

	* libgcc2.h (UBILtype): New typedef with may_alias attribute.
	(__mulbitint3, __divmodbitint4): Use UBILtype * instead of
	UWtype * and const UBILtype * instead of const UWtype *.
	* libgcc2.c (bitint_reduce_prec, bitint_mul_1, bitint_addmul_1,
	__mulbitint3, bitint_negate, bitint_submul_1, __divmodbitint4):
	Likewise.
	* soft-fp/bitint.h (UBILtype): Change define into a typedef with
	may_alias attribute.