fix signal return for sh/fdpic

the restorer function pointer provided in the kernel sigaction
structure is interpreted by the kernel as a raw code address, not a
function descriptor.

this commit moves the declarations of the __restore and __restore_rt
symbols to ksigaction.h so that arch versions of the file can override
them, and introduces a version for sh which declares them as objects
rather than functions.

an alternate solution would have been defining SA_RESTORER to 0 so
that the functions are not used, but this both requires executable
stack (since the sh kernel does not have a vdso page with permanent
restorer functions) and crashes on qemu user-level emulation.

add some comments about the mips ksigaction structure weirdness

fix breakage of x86_64 sigaction from recent changes for mips

use unsigned bitmask for consistency in ksigaction

the type doesn't actually matter, just the size, but it's nice to be
consistent...

fix breakage from last commit: forgot to include ksigaction.h

this file can be overridden by a same-named file in an arch dir.