reject crti.o/crtn.o, embrace the future

crti.o/crtn.o is a legacy strategy for calling constructor functions
upon object loading that has been superseded by the
init_array/fini_array mechanism.

Zig code depends on neither, since the language intentionally has no way
to initialize data at runtime, but alas the Zig linker still must
support this feature since popular languages depend on it.

Anyway, the way it works is that crti.o has the machine code prelude of
two functions called _init and _fini, each in their own section with the
respective name. crtn.o has the machine code instructions comprising the
exitlude for each function. In between, objects use the .init and .fini
link section to populate the function body.

This function is then expected to be called upon object initialization
and deinitialization.

This mechanism is depended on by libc, for example musl and glibc, but
only for older ISAs. By the time the libcs gained support for newer
ISAs, they had moved on to the init_array/fini_array mechanism instead.

For the Zig linker, we are trying to move the linker towards
order-independent objects which is incompatible with the legacy
crti/crtn mechanism.

Therefore, this commit drops support entirely for crti/crtn mechanism,
which is necessary since the other commits in this branch make it
nondeterministic in which order the libc objects and the other link
inputs are sent to the linker.

The linker is still expected to produce a deterministic output, however,
by ignoring object input order for the purposes of symbol resolution.

Author: andrewrk