xtensa: clean up functions in assembly code

Use ENTRY and ENDPROC throughout arch/xtensa/lib assembly sources.
Introduce asm/linkage.h and define xtensa-specific __ALIGN macro there.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

xtensa: Switch to asm-generic/linkage.h

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

xtensa: move headers files to arch/xtensa/include

Move all header files for xtensa to arch/xtensa/include and platform and
variant header files to the appropriate arch/xtensa/platforms/ and
arch/xtensa/variants/ directories.

Moving the files gets also rid of all uses of symlinks in the Makefile.

This has been completed already for the majority of the architectures
and xtensa is one out of six missing.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Chris Zankel <chris@zankel.net>