Bump GAS/BFD version to 2.15.91?

[Hans-Peter Nilsson]

Would it be ok to bump the BFD version, a.k.a. the GAS version
to 2.15.91?  I'm keying off the presence of the CRIS
-no-mul-bug-abort option for GCC on that version for in-tree-gas
(where GCC is configured before GAS and BFD is built, but GCC
configury peeks at the BFD version for assembler features, as
with all targets).  See gcc_GAS_CHECK_FEATURE usage in
gcc/configure.ac

brgds, H-P