We use Long_Long_Integer'Size / 2 instead of 4 * 8 to support
specifying a target configuration file where the largest integer is
32 bits instead of 64.

Tested on x86_64-pc-linux-gnu, committed on trunk

2019-09-17  Arnaud Charlet  <charlet@adacore.com>

gcc/ada/

	* libgnat/s-bitfie.ads (Val_Bits, Val_Bytes): Define from
	Long_Long_Integer'Size.