commit 8cf662ed upstream.

Old Microblaze toolchain supported "b" contstrains for
all register but it always points to general purpose reg.
New Microblaze toolchain is more strict in this
and general purpose register should be used there "r".
Signed-off-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>