libffi/test: Fix compilation for build sysroot

libffi/test: Fix compilation for build sysroot

Fix a problem with the libffi testsuite using a method to determine the
compiler to use resulting in the tool being different from one the
library has been built with, and causing a catastrophic failure from the
inability to actually choose any compiler at all in a cross-compilation
configuration.

Address this problem by providing a DejaGNU configuration file defining
the compiler to use, via the CC_FOR_TARGET TCL variable, set from $CC by
autoconf, which will have all the required options set for the target
compiler to build executables in the environment configured, removing
failures like:

FAIL: libffi.call/closure_fn0.c -W -Wall -Wno-psabi -O0 (test for excess errors)
Excess errors:
default_target_compile: No compiler to compile with
UNRESOLVED: libffi.call/closure_fn0.c -W -Wall -Wno-psabi -O0 compilation failed to produce executable

and bringing overall test results for the `riscv64-linux-gnu' target
(here with the `x86_64-linux-gnu' host and RISC-V QEMU in the Linux user
emulation mode as the target board) from:

		=== libffi Summary ===

# of unexpected failures	708
# of unresolved testcases	708
# of unsupported tests		30

to:

		=== libffi Summary ===

# of expected passes		1934
# of unsupported tests		28

This is a combined backport of the relevant parts of upstream libffi
changes as follows:

- commit 8308984e479e ("[PATCH] Make sure we're running dejagnu tests
  with the right compiler."),

- commit 2d9b3939751b ("[PATCH] Fix for closures with sunpro compiler"),

- commit 0c3824702d3d ("[PATCH] Always set CC_FOR_TARGET for dejagnu, to
  make the testsuite respect $CC"),

- commit 7d698125b1f0 ("[PATCH] Use the proper C++ compiler to run C++
  tests"),

- commit 6b6df1a7bb37 ("[PATCH] Adds `local.exp` to CLEANFILES"),

- commit 6cf0dea78a5a ("[PATCH] Change CLEANFILES to DISTCLEANFILES")

	libffi/
	* Makefile.am (DISTCLEANFILES): New variable.
	* configure.ac: Produce `local.exp'.
	* Makefile.in: Regenerate.
	* configure: Regenerate.
	* testsuite/Makefile.am (EXTRA_DEJAGNU_SITE_CONFIG): New
	variable.
	* testsuite/Makefile.in: Regenerate.