tidy m4 plugin config support

tidy m4 plugin config support

In CLANG_PLUGIN_FILE it is possible for plugin_file to be non-NULL
when LLVMgold.so does not exist.

configure output is messy, with results not printed against their
  "checking.." line, eg.
checking for clang... (cached) yes
checking for clang plugin file... checking for x86_64-pc-linux-gnu-ar... (cached) ar --plugin /usr/lib/llvm-20/lib/clang/20/../../LLVMgold.so
/usr/lib/llvm-20/lib/clang/20/../../LLVMgold.so

This patch fixes those problems, and a similar interposition of other
configure output between AC_MSG_CHECKING and AC_MSG_RESULT in
gcc-plugin.m4.  It also tidies some of the message text, and makes
similar code in gcc-plugin.m4 and clang-plugin.m4 a little more
consistent.

config/
	* clang-plugin.m4 (CLANG_PLUGIN_FILE): Don't place checks for
	tools (llvm-config, ar) inside AC_MSG_CHECKING..AC_MSG_RESULT
	for clang plugin file.  Clear plugin_file before loop exit.
	(CLANG_PLUGIN_FILE_FOR_TARGET): Similarly.
	* gcc-plugin.m4 (GCC_PLUGIN_OPTION): Similarly.
	(GCC_PLUGIN_OPTION_FOR_TARGET): Correct AC_MSG_CHECKING.  Tidy
	return code.
binutils/
	* testsuite/lib/binutils-common.exp <llvm_plug_opt>: Set for
	non-native.
	* configure: Regenerate.
/
	* configure: Regenerate.
bfd/
	* configure: Regenerate.
gas/
	* configure: Regenerate.
gdb/
	* configure: Regenerate.
gprof/
	* configure: Regenerate.
gprofng/
	* configure: Regenerate.
	* libcollector/configure: Regenerate.
ld/
	* configure: Regenerate.
libbacktrace/
	* configure: Regenerate.
libctf/
	* configure: Regenerate.
libiberty/
	* configure: Regenerate.
libsframe/
	* configure: Regenerate.
opcodes/
	* configure: Regenerate.
sim/
	* configure: Regenerate.
zlib/
	* configure: Regenerate.