arch/xtensa: Add automatic vector linkage generation

Existing solutions for linking the Xtensa vector table are a
cut-and-paste mess of inherited code, with more than a dozen special
sections that need to be linked into many special MEMORY{} regions.

Accept the existing convention used by C/asm code, but automatically
detect the needed offsets for the platform from core-isa.h (it can
share the preprocessing with gen_zsr.py) and emit a file that can be
included in lieu of all the existing boilerplate.

Signed-off-by: Andy Ross <andyross@google.com>