gdb/arm: fix IPSR field test in arm_m_exception_cache ()

Arm v8-M Architecture Reference Manual,
D1.2.141 IPSR, Interrupt Program Status Register reads
"Exception, bits [8:0]"

9 bits, not 8! It is uncommon but true!

Signed-off-by: Tomas Vanek <vanekt@fbl.cz>