powerpc/xmon: symbol lookup length fixed

Currently this cannot lookup symbol beyond 64 characters in some cases
like "ls", "lp" and "t"

Fix this by using KSYM_NAME_LEN instead of fixed 64 characters

Signed-off-by: Mukesh Kumar Chaurasiya <mchauras@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patch.msgid.link/20241024191232.1570894-2-mchauras@linux.ibm.com