Apply silent Makefile rules to gdb/testsuite

Apply silent Makefile rules to gdb/testsuite

This applies the silent-rules.mk treatment to gdb/testsuite/Makefile.

gdb/ChangeLog
2021-06-01  Tom Tromey  <tromey@adacore.com>

	* silent-rules.mk (ECHO_CC): New variable.

gdb/testsuite/ChangeLog
2021-06-01  Tom Tromey  <tromey@adacore.com>

	* Makefile.in (all): Don't print anything.
	($(abs_builddir)/site.exp site.exp): Use $(ECHO_GEN).
	(expect-read1): Likewise.
	(read1.so): Use $(ECHO_CC).
	Include silent-rules.mk.