[libc] [startup] add cmake function to merge separated crt1 objects (#75413)

As part of startup refactoring, this patch adds a function to merge
multiple objects into a single relocatable object:
                     cc -r obj1.o obj2.o -o obj.o

A relocatable object is an object file that is not fully linked into an
executable or a shared library. It is an intermediate file format that
can be passed into the linker.

A crt object can have arch-specific code and arch-agnostic code. To
reduce code cohesion, the implementation is splitted into multiple
units. As a result, we need to merge them into a single relocatable
object.