native_interpret_real in fold_const.c has an assumption that floats are at least 
32-bits (on bigendian targets with UNITS_PER_WORD >= 4). This patch relaxes that 
assumption (allowing e.g. 16-bit HFmode values).

On aarch64_be-none-elf, this fixes the float16x4_t variant of 
gcc.target/aarch64/advsimd-intrinsics/vcreate.c added in the next patch in series.

Bootstrapped + check-gcc on:
x86-unknown-linux-gnu
powerpc64-unknown-linux-gnu (gcc110 on compile farm, as I believe this is 
bigendian, as opposed to powerpc64le-unknown-linux-gnu on gcc112)
arm-none-linux-gnueabihf
aarch64-none-elf

It's not clear that any of those actually test this code, but the 
aarch64_be-none-elf tests in next patch definitely do ;).

gcc/ChangeLog:

	fold-const.c (native_interpret_real): Correctly read floats of less
	than 32 bits on bigendian targets.