CMake Build: Tidy up when -msse2 is passed to gcc (#622)

* Define CMAKE_ARM_COMPILER automatically (#469)

Makes use of CMAKE_SYSTEM_PROCESSOR to detect whether compilation is happening for an ARM architecture.

* Use the correct guard for specifying msse2 compiler flags.

Bruteforce and conversions were only checking against ANDROID which was not sufficient for ARM Linux builds.

* Remove redundant uses of the msse2 compiler flag.

* Globally set -msse2 for x86/amd64 gcc, Apple and Clang builds.

This is a change in behaviour. Prior to this commit, msse2 was only set for bruteforce and conversions.

msse and mfpmath=sse were already set globally prior to this change.

test_conformance/thread_dimensions/CMakeLists.txt

@@ -5,7 +5,4 @@ set(${MODULE_NAME}_SOURCES
     test_thread_dimensions.cpp
 )
 
-set_source_files_properties(
-        COMPILE_FLAGS -msse2)
-
 include(../CMakeCommon.txt)