Darwin, Simplify headers 0/5 : Move spec for Darwin 10 unwind stub [NFC].

Darwin, Simplify headers 0/5 : Move spec for Darwin 10 unwind stub [NFC].

The darwinN.h headers were (presumably) introduced to allow specs to be
adjusted when there was no mmacosx-version-min handling, or that was
considered unreliable.

We have version-specific specs for the values that have configuration
data, and the version is set in the driver (so may be considered
reliably present).

Some of the 'darwinN.h' content has become dead code, and the reminder
is either conditionalised on version information (or is setting values
used as fall-backs in cross-compilations).

With the changes needed for Darwin20 / macOS 11 the 'darwnN.h' headers
are now too unwieldy to be useful - so this series moves the relevant
specs definitons to the common 'darwin.h' header and then finally uses
the config.gcc script to supply the fall-back defaults for cross-
compilations.

We can then delete all but the main header, since the darwinN.h are
unused.

This change moves a spec from darwin10.h to the main darwin.h
target header.

gcc/ChangeLog:

	* config/darwin10.h (LINK_GCC_C_SEQUENCE_SPEC): Move the spec
	for the Darwin10 unwinder stub from here ...
	* config/darwin.h (LINK_COMMAND_SPEC_A): ... to here.