This patch lets MoarVM build against libtommath

https://github.com/libtom/libtommath/pull/476

Adjusted to work also on PowerPC

--- bn_mp_set_double.c.orig
+++ bn_mp_set_double.c
@@ -3,7 +3,7 @@
 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
 /* SPDX-License-Identifier: Unlicense */
 
-#if defined(__STDC_IEC_559__) || defined(__GCC_IEC_559)
+#if defined(MP_HAS_SET_DOUBLE)
 mp_err mp_set_double(mp_int *a, double b)
 {
    uint64_t frac;
--- tommath_private.h.orig
+++ tommath_private.h
@@ -164,6 +164,14 @@
 
 MP_STATIC_ASSERT(prec_geq_min_prec, MP_PREC >= MP_MIN_PREC)
 
+#if defined(__STDC_IEC_559__) || defined(__GCC_IEC_559) \
+   || defined(__x86_64__) || defined(_M_X64) || defined(_M_AMD64) \
+   || defined(__i386__) || defined(_M_X86) \
+   || defined(__POWERPC__) \
+   || defined(__aarch64__) || defined(__arm__)
+#define MP_HAS_SET_DOUBLE
+#endif
+
 /* random number source */
 extern MP_PRIVATE mp_err(*s_mp_rand_source)(void *out, size_t size);