mips/abi64.h SETUP_INCOMING_VARARGS vs tm.texi

[Joern Rennecke <amylaar at cygnus dot co dot uk>]

tm.texi says:

Because you must generate code to push the anonymous arguments at
compile time without knowing their data types,
@code{SETUP_INCOMING_VARARGS} is only useful on machines that have just
a single category of argument register and use it uniformly for all data
types.

however, config/mips/abi64.h defines SETUP_INCOMING_VARARGS and uses it
for a family of ABIs that use two different categories of argument passing
registers: general purpose and floating point registers.

Either tm.texi is wrong, or abi64.h does something that is not really
supported and could break in the future, e.g. when somebody adds bounds
checking to varargs accesses in the SETUP_INCOMING_VARARGS case.