[gdb/testsuite] Handle early_flags in gdb_default_target_compile

In gdb_default_target_compile, we use dejagnu's default_target_compile, unless
we need support for languages that are not yet support in the used dejagnu
version, in which case we use a local default_target_compile:
gdb_default_target_compile_1.

However, there's another reason to use the local default_target_compile: when
early_flags is used, because there's no dejagnu release available yet
supporting this.

Fix this by detecting and handling early_flags in gdb_default_target_compile.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-06-30  Tom de Vries  <tdevries@suse.de>

	PR testsuite/26175
	* lib/future.exp (gdb_default_target_compile): Detect and handle
	early_flags.