[gdb/tdep] Fix ARM_LINUX_JB_PC_EABI

In arm-linux-tdep.c, ARM_LINUX_JB_PC_EABI is defined as 9, but it's been 1
since glibc 2.20.

See glibc commit 80a56cc3ee ("ARM: Add SystemTap probes to longjmp and
setjmp.").

Update it, allowing us to run into the gdb/26967 kfail.

Tested on arm-linux.

Approved-By: Luis Machado <luis.machado@arm.com>

PR arm/tdep
Bug: https://www.sourceware.org/bugzilla/show_bug.cgi?id=31089