Port a lot more code to AARCH64

- Introduce epoll_pwait()
- Rewrite -ftrapv and ffs() libraries in C code
- Use more FreeBSD code in math function library
- Get significantly more tests passing on qemu-aarch64
- Fix many Musl long double functions that were broken on AARCH64

dsp/core/sad16x8n.c

@@ -0,0 +1,46 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2023 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "dsp/core/core.h"
+#include "libc/limits.h"
+#include "libc/macros.internal.h"
+#include "third_party/aarch64/arm_neon.h"
+#include "third_party/intel/emmintrin.internal.h"
+
+/**
+ * Mixes audio.
+ *
+ * This function performs saturated addition on an array of shorts.
+ *
+ * @param x needs to be 16-byte aligned
+ * @param y needs to be 16-byte aligned
+ */
+void sad16x8n(size_t n, short x[n][8], const short y[n][8]) {
+  size_t i, j;
+  for (i = 0; i < n; ++i) {
+#ifdef __x86_64__
+    *(__m128i *)x[i] = _mm_adds_epi16(*(__m128i *)x[i], *(__m128i *)y[i]);
+#elif defined(__aarch64__)
+    *(int16x4_t *)x[i] = vqadd_s16(*(int16x4_t *)x[i], *(int16x4_t *)y[i]);
+#else
+    for (j = 0; j < 8; ++j) {
+      x[i][j] = MIN(MAX(x[i][j] + y[i][j], INT16_MIN), INT16_MAX);
+    }
+#endif
+  }
+}