Fix building on macOS hosts

Explicitly set CMake to use a 'Generic' target so that it does
not pass any macOS specific compiler flags (e.g -isysroot) to
our embedded aarch64-none-elf- toolchain that does not support
them.

CMakeLists.txt

@@ -2,6 +2,8 @@ cmake_minimum_required(VERSION 3.2)
 
 set(MUSL $ENV{MUSL})
 
+set(CMAKE_SYSTEM_NAME Generic)
+
 set(CMAKE_C_COMPILER aarch64-none-elf-gcc)
 set(CMAKE_CXX_COMPILER aarch64-none-elf-g++)
 set(CMAKE_C_COMPILER_WORKS 1)