tune-cortexa32: set tune feature as armv8a

tune-cortexa32: set tune feature as armv8a

Cortexa32 is a 32-bit armv8a architecture processor, so set the tune feature
as armv8a instead of aarch64 which is 64-bit armv8a architecture.

It solves the following build error while compiling libgcc-initial
and libssp-nonshared.
-- snip --
aarch64-poky-linux-musl-gcc: error: unrecognized command-line option '-mfpu=neon'
aarch64-poky-linux-musl-gcc: error: unrecognized command-line option '-mfloat-abi=hard'
-- snip --

Signed-off-by: Jagadeesh Krishnanjanappa <workjagadeesh@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>