arch-arm*: add better support for gcc march extensions

GCC has the ability to pass extensions to the march parameter, which
expand the funcationality of the march.  For example
"-march=armv7ve+simd" adds SIMD to ARMv7.  Currently, there is no way to
expand the march setting without modifying each instance, as you can't
guarantee the ordering when using the existing TUNE_CCARGS.  By
introducing two new variables, TUNE_CCARGS_MARCH and
TUNE_CCARGS_MARCH_OPT, we can enforce that these two go together.

Also, expand existing and create new feature files that use these
variables to expand the functionality.

Signed-off-by: Jon Mason <jdmason@kudzu.us>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>