fix: do not force HARDFLOAT when using soft fp (#8)

Fixes compilation on Debian armel, see
https://buildd.debian.org/status/fetch.php?pkg=vixl&arch=armel&ver=5.1.0-1&stamp=1637517716&raw=0

Previously, Vixl would force hard float on all arm CPUs, by checking
only for __arm__. This would cause GCC to throw an internal error when
compiling with -mfloat-abi=soft. See also this example on Compiler
Explorer: https://compiler-explorer.com/z/zsn5vaMna