DML uses the compiler option -mpreferred-stack-boundary=4 to configure
a stack alignment of 16 bytes. Clang uses the option -mstack-alignment
instead, which expects as parameter the alignment in bytes, and not a
power of two like -mpreferred-stack-boundary.

Probe for both compiler options and use the correct one, similar to
what is done in arch/x86/Makefile.
Reported-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>