avutil/mips/mmiutils: provide the Loongson MMI mnemonics GNU binutils lacks

GNU binutils spells the MMI instructions pxor, pand, por, pnor, ssrld
and sslld as xor, and, or, nor, dsrl and dsll on floating point
registers, with -march=loongson2e/2f/3a and since 2.32 with
-mloongson-mmi; the p-named entries in its opcode table are the R5900
instructions on general purpose registers. The other 43 p-named MMI
instructions FFmpeg uses are spelled the same by everyone. Loongson's
gcc toolchain accepts both spellings (ebedd26eef), the binutils 2.24
of their cross-gcc-4.9.3-n64-loongson-rc6.1 only the GNU ones.

Since 903c5d58f0 configure probes pxor and so silently disables MMI
with every GNU binutils. Probe Loongson's spelling first and fall
back to the GNU one together with assembler macros that define the
six names, as asmdefs.h already does for parse_r. The macros assemble
to the same encodings as the GNU spelling with binutils 2.24, 2.28
and 2.42.

The GNU fallback is limited to little-endian mips64: the MMI code does
not build for O32 and its unaligned loads assume little-endian.

configure

@@ -2597,6 +2597,7 @@ TOOLCHAIN_FEATURES="
     ibm_asm
     inline_asm_direct_symbol_refs
     inline_asm_nonlocal_labels
+    loongson_mmi_mnemonics
     pragma_deprecated
     rsync_contimeout
     symver_asm_label
@@ -6658,7 +6659,14 @@ elif enabled mips; then
     enabled loongson3 && check_inline_asm loongson3 '"gsldxc1 $f0, 0($2, $3)"' '-mloongson-ext' && append MMIFLAGS '-mloongson-ext'
 
     # MMI can be detected at runtime too
-    enabled mmi && check_inline_asm mmi '"pxor $f0, $f0, $f0"' '-mloongson-mmi' && append MMIFLAGS '-mloongson-mmi'
+    enabled mmi && {
+        check_inline_asm loongson_mmi_mnemonics '"pxor $f0, $f0, $f0\n\tssrld $f0, $f0, $f0"' '-mloongson-mmi' || {
+            # the MMI code does not build for O32 and assumes little-endian
+            disable mmi
+            enabled mips64 && ! enabled bigendian &&
+                check_inline_asm mmi '"xor $f0, $f0, $f0"' '-mloongson-mmi'
+        }
+    } && append MMIFLAGS '-mloongson-mmi'
 
     if enabled bigendian && enabled msa; then
         disable msa

libavutil/mips/mmiutils.h

@@ -29,6 +29,16 @@
 #include "libavutil/mem_internal.h"
 #include "libavutil/mips/asmdefs.h"
 
+#if !HAVE_LOONGSON_MMI_MNEMONICS
+/* GNU binutils lacks Loongson's names for these MMI instructions */
+__asm__(".macro pxor  d, s, t\n\txor  \\d, \\s, \\t\n\t.endm\n"
+        ".macro pand  d, s, t\n\tand  \\d, \\s, \\t\n\t.endm\n"
+        ".macro por   d, s, t\n\tor   \\d, \\s, \\t\n\t.endm\n"
+        ".macro pnor  d, s, t\n\tnor  \\d, \\s, \\t\n\t.endm\n"
+        ".macro ssrld d, s, t\n\tdsrl \\d, \\s, \\t\n\t.endm\n"
+        ".macro sslld d, s, t\n\tdsll \\d, \\s, \\t\n\t.endm\n");
+#endif
+
 /*
  * These were used to define temporary registers for MMI marcos
  * however now we're using $at. They're theoretically unnecessary