Downgrade riscv64-specific LTO error to a warning

Downgrade riscv64-specific LTO error to a warning

Error happens when mixing some PIC and non-PIC code in an LTO build/link

  ld: error: linking module flags 'SmallDataLimit': IDs have conflicting values in '<REDACTED>.o' and 'ld-temp.o'

and affects a few ports now.  Issue reported upstream where the proposed
fix uses llvm::Module::Min, which we don't have and would require
a backport.  For now, work around this issue by downgrading to
a warning, which should have the intended effect in most cases (use the
value of the first module, which is smaller than the defaults value used
by ld-temp.o).

ok kettenis@