Add C_FLAGS, CPP_FLAGS and ASM_FLAGS variables to the build system BUILD module to allow for language-specific compiler/assembler flags.

Oops - fix accidental negation in the BUILD module architecture detection code. Add error if an unsupported architecture is requested.

Disable linker relaxations for the UC3 architecture, as this does not appear to be supported in the linker.

Replace architecture specific makefiles in the BuildTests with the new common build system makefiles.