build: Check for ld -z compress-sections=zstd on Solaris

build: Check for ld -z compress-sections=zstd on Solaris

The Solaris 11.4 ld recently gained support for zstd compression.  This
patch adds a configure check for it.

I also noticed that we don't currently document -gz=zstd, so this patch
adds that, too.

Tested on i386-pc-solaris2.11 (with pre-zstd and post-zstd ld) and
x86_64-pc-linux-gnu.

2025-11-14  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	gcc:
	* configure.ac (gcc_cv_ld_compress_debug) <*-*-solaris2*>: Check
	for zstd compression support.
	* configure: Regenerate.

	* doc/invoke.texi (Debugging Options, gz): Document zstd.