toolchain: Replace GCC_VERSION, CLANG_VERSION and BUILD_ASSERT macros

GCC_VERSION is defined in a few modules, and those headers are often
included first, so replace the one used in zephyr with
TOOLCHAIN_GCC_VERSION. Do the same with CLANG_VERSION, replacing it with
TOOLCHAIN_CLANG_VERSION.

BUILD_ASSERT is also defined in include/toolchain/common.h, which might
get included before gcc.h. We want to use the gcc-specific one instead
of the general one.

Signed-off-by: Keith Packard <keithp@keithp.com>