Update BUILD.gn configs to support ARM[64] builds.

Bug: b/114402930
Change-Id: Id7eedc6b01f40d24188d514755d5c89689b24abb
Reviewed-on: https://swiftshader-review.googlesource.com/20568
Tested-by: Stephen Lanham <slan@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
diff --git a/src/Reactor/SubzeroReactor.cpp b/src/Reactor/SubzeroReactor.cpp
index 3d49658..0103a25 100644
--- a/src/Reactor/SubzeroReactor.cpp
+++ b/src/Reactor/SubzeroReactor.cpp
@@ -103,7 +103,7 @@
 
 		static bool detectARM()
 		{
-			#if defined(__arm__)
+			#if defined(__arm__) || defined(__aarch64__)
 				return true;
 			#elif defined(__i386__) || defined(__x86_64__)
 				return false;
@@ -372,6 +372,8 @@
 			assert(sizeof(void*) == 8 && elfHeader->e_machine == EM_X86_64);
 		#elif defined(__arm__)
 			assert(sizeof(void*) == 4 && elfHeader->e_machine == EM_ARM);
+		#elif defined(__aarch64__)
+			assert(sizeof(void*) == 8 && elfHeader->e_machine == EM_AARCH64);
 		#else
 			#error "Unsupported platform"
 		#endif