[gdb/testsuite] Handle missing gccgo

Without gccgo installed I see in stdout/stderr:
...
Running /data/gdb_versions/devel/src/gdb/testsuite/gdb.go/print.exp ...
Running /data/gdb_versions/devel/src/gdb/testsuite/gdb.go/handcall.exp ...
gdb compile failed, default_target_compile: Can't find gccgo.
Running /data/gdb_versions/devel/src/gdb/testsuite/gdb.go/max-depth.exp ...
gdb compile failed, default_target_compile: Can't find gccgo.
Running /data/gdb_versions/devel/src/gdb/testsuite/gdb.go/integers.exp ...
gdb compile failed, default_target_compile: Can't find gccgo.
Running /data/gdb_versions/devel/src/gdb/testsuite/gdb.go/unsafe.exp ...
gdb compile failed, default_target_compile: Can't find gccgo.
Running /data/gdb_versions/devel/src/gdb/testsuite/gdb.go/package.exp ...
gdb compile failed, default_target_compile: Can't find gccgo.
Running /data/gdb_versions/devel/src/gdb/testsuite/gdb.go/types.exp ...
gdb compile failed, default_target_compile: Can't find gccgo.
Running /data/gdb_versions/devel/src/gdb/testsuite/gdb.go/chan.exp ...
gdb compile failed, default_target_compile: Can't find gccgo.
Running /data/gdb_versions/devel/src/gdb/testsuite/gdb.go/strings.exp ...
gdb compile failed, default_target_compile: Can't find gccgo.
Running /data/gdb_versions/devel/src/gdb/testsuite/gdb.go/basic-types.exp ...
Running /data/gdb_versions/devel/src/gdb/testsuite/gdb.go/hello.exp ...
gdb compile failed, default_target_compile: Can't find gccgo.
Running /data/gdb_versions/devel/src/gdb/testsuite/gdb.go/methods.exp ...
gdb compile failed, default_target_compile: Can't find gccgo.
...

Fix this by introducing a gdb_caching_proc support_go_compile, and using it in
the complaining test-cases.

Tested on x86_64-linux, with and without gccgo installed.

gdb/testsuite/ChangeLog:

2020-02-20  Tom de Vries  <tdevries@suse.de>

	* lib/gdb.exp (support_go_compile): New gdb_caching_proc.
	(gdb_simple_compile): Handle compile_flags go by using .go extension
	for source file.
	* gdb.go/chan.exp: Use support_go_compile.
	* gdb.go/handcall.exp: Same.
	* gdb.go/hello.exp: Same.
	* gdb.go/integers.exp: Same.
	* gdb.go/max-depth.exp: Same.
	* gdb.go/methods.exp: Same.
	* gdb.go/package.exp: Same.
	* gdb.go/strings.exp: Same.
	* gdb.go/types.exp: Same.
	* gdb.go/unsafe.exp: Same.