tcg: Return success from patch_reloc

tcg: Return success from patch_reloc

This will move the assert for success from within (subroutines of)
patch_reloc into the callers.  It will also let new code do something
different when a relocation is out of range.

For the moment, all backends are trivially converted to return true.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>