driver: Spill long COLLECT_GCC_OPTIONS to a response file [PR111527]

Many kernels enforce a per-string length limit on argv and envp
strings passed to execve().  On Linux, MAX_ARG_STRLEN limits each
string to 32 * PAGE_SIZE (~128KB); Windows limits individual
environment variables to 32767 characters.  When the assembled
value exceeds such a limit, the build fails.

When the assembled value would exceed COLLECT2_OPTIONS_MAX_LENGTH
(default 1024, host-overridable via defaults.h), the driver writes
the option list to a temporary response file via writeargv() and
exports "COLLECT_GCC_OPTIONS=@<path>" instead.  collect2,
lto-wrapper and lto-plugin transparently expand the @file form using
existing expandargv() infrastructure, so the change is invisible to
normal builds.

Bootstrapped and regression tested on x86_64-pc-linux-gnu.

	PR driver/111527

gcc/ChangeLog:

	* defaults.h (COLLECT2_OPTIONS_MAX_LENGTH): New macro.
	* collect-utils.cc (read_collect_gcc_options): New function.
	* collect-utils.h (read_collect_gcc_options): Declare.
	* collect2.cc (main): Use read_collect_gcc_options instead
	of getenv.
	* doc/hostconfig.texi (Host Misc): Document
	COLLECT2_OPTIONS_MAX_LENGTH.
	* doc/invoke.texi (Environment Variables): Document the
	@file form of COLLECT_GCC_OPTIONS.
	* gcc.cc (xsetenv_collect_gcc_options): New function.
	(set_collect_gcc_options): Use xsetenv_collect_gcc_options.
	* lto-wrapper.cc (run_gcc): Use read_collect_gcc_options
	instead of getenv.

gcc/testsuite/ChangeLog:

	* gcc.misc-tests/pr111527.exp: New test.

include/ChangeLog:

	* libiberty.h (expandargstr): Declare.

libiberty/ChangeLog:

	* argv.c (expandargstr): New function.

lto-plugin/ChangeLog:

	* lto-plugin.c (read_collect_gcc_options): New function.
	(onload): Use read_collect_gcc_options instead of getenv.

Signed-off-by: Sunil Dora <sunilkumar.dora@windriver.com>