Fix Android FIPS build on armv8 (#299)

### Issues:
Resolves #192

### Description of changes: 
* Clang with the Android NDK doesn't compile with `-Werror` on clang due to implicit addition of `-Wa,noexecstack`. Since `--Wa,--noexecstack` is not used during the preprocessor step (because assembler is not invoked), Clang reports that argument as unused. Added a removal the flag during FIPS builds for Android. 
Context: android/ndk#171

* CMAKE inserts a `\` before whitespaces in arguments, which Android line does not recognize when using
  `add_custom_command`. This caused `${CMAKE_ASM_COMPILER}` to fail during the preprocessing step. Replacing whitespaces with `;` fixes this. 
  Context: https://stackoverflow.com/questions/8925396/why-does-cmake-prefixes-spaces-with-backslashes-when-executing-a-command

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

crypto/fipsmodule/CMakeLists.txt

@@ -16,6 +16,19 @@ if(FIPS_SHARED AND FIPS_VENDOR_AFFIRM_BCM_O_PATH)
   return()
 endif()
 
+if(ANDROID)
+  # Since "--Wa,--noexecstack" is not used during the preprocessor step of Android (because assembler is not invoked),
+  # Clang reports that argument as unused. We remove the flag only for the FIPS build of Android.
+  string(FIND ${CMAKE_CXX_FLAGS} "noexecstack" CXX_EXTRA_WA)
+  string(FIND ${CMAKE_C_FLAGS} "noexecstack" C_EXTRA_WA)
+  if(NOT ${CXX_EXTRA_WA} EQUAL '-1')
+    string( REPLACE "-Wa,--noexecstack" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}" )
+  endif()
+  if(NOT ${C_EXTRA_WA} EQUAL '-1')
+    string( REPLACE "-Wa,--noexecstack" "" CMAKE_C_FLAGS "${CMAKE_C_FLAGS}" )
+  endif()
+endif()
+
 if(ARCH STREQUAL "x86_64")
   set(
     BCM_ASM_SOURCES
@@ -152,9 +165,10 @@ function(cpreprocess dest src)
     set(TARGET "--target=${CMAKE_ASM_COMPILER_TARGET}")
   endif()
 
+  string(REGEX REPLACE "[ ]+" ";" CMAKE_ASM_FLAGS "${CMAKE_ASM_FLAGS}")
   add_custom_command(
     OUTPUT ${dest}
-    COMMAND ${CMAKE_ASM_COMPILER} ${TARGET} $CMAKE_ASM_FLAGS -E ${src} -I${PROJECT_SOURCE_DIR}/include > ${dest}
+    COMMAND ${CMAKE_ASM_COMPILER} ${TARGET} ${CMAKE_ASM_FLAGS} -E ${src} -I${PROJECT_SOURCE_DIR}/include > ${dest}
     DEPENDS
     ${src}
     ${PROJECT_SOURCE_DIR}/include/openssl/arm_arch.h