toolchain: introduce macros to ignore -Wshadow

This introduces two macros TOOLCHAIN_IGNORE_WSHADOW_BEGIN and
TOOLCHAIN_IGNORE_WSHADOW_END which can be used inside another
macro to ignore -Wshadow for certain block of code. This is
useful for common macros that may nest upon themselves
(for example, logging).

Signed-off-by: Daniel Leung <daniel.leung@intel.com>