Go back to using compiler built-in macros in ARM assembly code

These files don't #include SDL headers, so SDL-specific macros will
never be defined here.

This partially reverts commit 31d133db.

Fixes: 31d133db "Define SDL_PLATFORM_* macros instead of underscored ones (#8875)"
Signed-off-by: Simon McVittie <smcv@collabora.com>

src/video/arm/pixman-arm-neon-asm.S

@@ -44,7 +44,7 @@
  */
 
 /* Prevent the stack from becoming executable for no reason... */
-#if defined(SDL_PLATFORM_LINUX) && defined(__ELF__)
+#if defined(__linux__) && defined(__ELF__)
 .section .note.GNU-stack,"",%progbits
 #endif
 

src/video/arm/pixman-arm-simd-asm.S

@@ -19,7 +19,7 @@
  */
 
 /* Prevent the stack from becoming executable */
-#if defined(SDL_PLATFORM_LINUX) && defined(__ELF__)
+#if defined(__linux__) && defined(__ELF__)
 .section .note.GNU-stack,"",%progbits
 #endif