xtensa: add asm-prototypes.h

Move assembly source prototypes from xtensa_ksyms.c to
asm/asm-prototypes.h, move corresponding EXPORT_SYMBOLs to the assembly
sources and enable HAVE_ASM_MODVERSIONS for xtensa.

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

xtensa: implement call0 ABI support in assembly

Replace hardcoded register and opcode names with ABI-agnostic macros.
Add register save/restore code where necessary. Conditionalize windowed
only or call0 only code. Add stack initialization matching _switch_to
epilogue to copy_thread.

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

xtensa: abstract 'entry' and 'retw' in assembly code

Provide abi_entry, abi_entry_default, abi_ret and abi_ret_default macros
that allocate aligned stack frame in windowed and call0 ABIs.
Provide XTENSA_SPILL_STACK_RESERVE macro that specifies required stack
frame size when register spilling is involved.
Replace all uses of 'entry' and 'retw' with the above macros.
This makes most of the xtensa assembly code ready for XEA3 and call0 ABI.

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

xtensa: add static function tracer support

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>