[gdb/testsuite] Fix gdb.threads/ia64-sigill.exp with check-read1

Fix test-case gdb.threads/ia64-sigill.exp with make target check-read1 by
using a custom line-by-line exp_continue clause:
...
    -re "\r\n\[^\r\n\]*(?=\r\n\[^\r\n\]*\r\n)" {
	exp_continue
    }
...
which drops a line each time it finds two lines in the buffer.

This allows the other clauses to use two-line patterns.

Tested on x86_64-linux.