[gdb/testsuite] Add -early pattern flag for gdb_test_multiple

[gdb/testsuite] Add -early pattern flag for gdb_test_multiple

Proc gdb_test_multiple builds up and executes a gdb_expect expression with
pattern/action clauses.  The clauses are either implicit (added by
gdb_test_multiple) or explicit (passed via the gdb_test_multiple parameter
user_code).

However, there are a few implicit clauses which are inserted before the
explicit ones, making sure those take precedence.

Add an -early pattern flag for a gdb_test_multiple user_code clause to specify
that the clause needs to be inserted before any implicit clause.

Using this pattern flag, we can f.i. setup a kfail for an assertion failure
<assert> during gdb_continue_to_breakpoint by the rewrite:
...
gdb_continue_to_breakpoint <msg> <pattern>
...
into:
...
set breakpoint_pattern "(?:Breakpoint|Temporary breakpoint) .* (at|in)"
gdb_test_multiple "continue" "continue to breakpoint: <msg>"  {
   -early -re "internal-error: <assert>" {
       setup_kfail gdb/nnnnn "*-*-*"
       exp_continue
   }
   -re "$breakpoint_pattern <pattern>\r\n$gdb_prompt $" {
       pass $gdb_test_name
   }
}

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-10-30  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (gdb_test_multiple): Handle -early pattern flag.

Change-Id: I376c636b0812be52e7137634b1a4f50bf2b999b6