[gdb/testsuite] Fix gdb.base/dcache-flush.exp

When running test-case gdb.base/dcache-flush.exp on ubuntu 18.04.5, I run into:
...
(gdb) PASS: gdb.base/dcache-flush.exp: p var2
info dcache^M
Dcache 4096 lines of 64 bytes each.^M
Contains data for Thread 0x7ffff7fc6b80 (LWP 3551)^M
Line 0: address 0x7fffffffd4c0 [47 hits]^M
Line 1: address 0x7fffffffd500 [31 hits]^M
Line 2: address 0x7fffffffd5c0 [7 hits]^M
Cache state: 3 active lines, 85 hits^M
(gdb) FAIL: gdb.base/dcache-flush.exp: check dcache before flushing
...
The regexp expects "Contains data for process $decimal".

This is another case of thread_db_target::pid_to_str being used.

Fix this by updating the regexp.

Tested on x86_64-linux.