Don't use yield opcode if unsupported.

This is an enhancement to the previous `yield` work:

* Don't enforce `-march=armv8-a` for aarch64 builds, because it is the
  initial ARMv8 revision and compilers will either use that or something
  newer.
* Refine preprocessor guards around `asm("yield");` so the code isn't
  compiled in if unsupported by the current `-march='.

Submitted by @smcv in #535.

CMakeLists.txt

@@ -168,8 +168,6 @@ endif()
 if ("${ARCH}" STREQUAL "arm")
     if (CMAKE_SIZEOF_VOID_P EQUAL 4)
         set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -march=armv6k")
-    else()
-        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -march=armv8-a")
     endif()
 endif()
 

Makefile

@@ -152,16 +152,12 @@ endif
 # is also the first Raspberry PI generation and older hardware
 # is likely too slow to run the game. We're not enforcing the
 # minimum architecture, but if you're build for something older
-# like arm5 you need to remove the `yield` opcode. It's set in
-# Qcommon_Mainloop() in frame.c via inline assmebly.
+# like arm5 the `yield` opcode isn't compiled in and the game
+# (especially q2ded) will consume more CPU time than necessary.
 ifeq ($(YQ2_ARCH), arm)
 CFLAGS += -march=armv6k
 endif
 
-ifeq ($(YQ2_ARCH), aarch64)
-CFLAGS += -march=armv8.0-a
-endif
-
 # ----------
 
 # Switch of some annoying warnings.

src/common/frame.c

@@ -146,7 +146,7 @@ Qcommon_Mainloop(void)
 				   a Kaby Lake laptop. */
 #if defined (__GNUC__) && (__i386 || __x86_64__)
 				asm("pause");
-#elif defined(__arm__) || defined(__aarch64__)
+#elif defined(__aarch64__) || (defined(__ARM_ARCH) && __ARM_ARCH >= 7) || defined(__ARM_ARCH_6K__)
 				asm("yield");
 #endif