Fix some spelling errors.

I noticed those from a lintian run:
https://salsa.debian.org/cbiesinger-guest/gdb/-/jobs/514119

gdb/ChangeLog:

2020-01-16  Christian Biesinger  <cbiesinger@google.com>

        * btrace.c (btrace_compute_ftrace_1): Fix spelling error (Unkown).
        (btrace_stitch_trace): Likewise.
        * charset.c (intermediate_encoding): Likewise (vaild).
        * nat/linux-btrace.c (linux_read_pt): Likewise (Unkown).
        * python/py-record-btrace.c (struct PyMethodDef): Likewise (occurences).
        * record-btrace.c (record_btrace_print_conf): Likewise (unkown).

gdb/testsuite/ChangeLog:

2020-01-16  Christian Biesinger  <cbiesinger@google.com>

        * lib/gdb.exp: Fix spelling error (seperatelly).

Change-Id: I2a44936bac295020f217fb6c78b99b0a8d09cf9a