xtensa: Makefile: cleanup

Some options which are already defined by the build system
for the C compiler or assembler have been removed: -c,
-xassembler-with-cpp, -nostdinc.
References to deleted variable flagALongCall and flagLongCall
removed.
Formatting for 80 columns, there is now one flag per line.

Change-Id: Ieecdb75e26f64c6f58dec3cc636552e7b31a678d
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>