[libc][cmake] append per obj compile options instead of prepending (#77126)

This allows individual object files to override the common compile
commands in
their local CMakeLists' add_object_library call.

For example, the common compile commands contain -Wall and -Wextra.
Before
this patch, the per object COMPILE_OPTIONS were prepended to these, so
that
builds of individual object files could not individually disable
specific
diagnostics from those groups explicitly.

After this patch, the per-object file compile objects are appended to
the list
of compiler flags, enabling this use case.

ARGN is a bit of cmake magic; let's be explicit in the APPEND that we're
appending the compile options.

Link: #77007