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

src/aarch32/instructions-aarch32.h

@@ -38,7 +38,7 @@ extern "C" {
 #include "utils-vixl.h"
 #include "aarch32/constants-aarch32.h"
 
-#ifdef __arm__
+#if defined(__arm__) && !defined(__SOFTFP__)
 #define HARDFLOAT __attribute__((noinline, pcs("aapcs-vfp")))
 #else
 #define HARDFLOAT __attribute__((noinline))