rjpeg: detect __ARM_NEON in addition to __ARM_NEON__

Modern aarch64-linux-gnu-gcc defines __ARM_NEON (no trailing
underscore) but not __ARM_NEON__. rjpeg's auto-detect at line 189
tested __ARM_NEON__ only, so modern aarch64 builds silently ran
the scalar kernel unless the user passed -D__ARM_NEON__ or
-DHAVE_NEON explicitly.

rpng at libretro-common/formats/png/rpng.c:38 already accepts both
spellings:

  #elif defined(__ARM_NEON) || defined(__ARM_NEON__)

Match that. One real change (add `defined(__ARM_NEON) ||`);
the #if is split across two lines because the condition grew long.

Verified on an aarch64 cross-compile with no explicit
-D__ARM_NEON__: with the fix, rjpeg now selects its NEON kernels
automatically. Reproduced the old behaviour (scalar fallback) by
temporarily reverting this one-line change and confirming that
sabotaging NEON kernels has no effect on test output -- the NEON
code was unreachable. With the fix, the same sabotage correctly
causes test failures, proving the NEON kernels are now entered
from `__ARM_NEON` alone.

Author: LibretroAdmin

libretro-common/formats/jpeg/rjpeg.c

@@ -186,7 +186,8 @@ struct rjpeg
 #endif
 
 /* Auto-detect NEON support */
-#if !defined(RJPEG_NO_SIMD) && !defined(RJPEG_NEON) && (defined(__ARM_NEON__) || defined(HAVE_NEON))
+#if !defined(RJPEG_NO_SIMD) && !defined(RJPEG_NEON) \
+   && (defined(__ARM_NEON) || defined(__ARM_NEON__) || defined(HAVE_NEON))
 #define RJPEG_NEON
 #endif