sys-libs/glibc: workaround gawk slowness in tests

It's not clear if glibc is doing something odd here in the tests or
if >=gawk-5.4.0 is to blame with the new matchers it has. Revert to the GNU
matchers for now for the testsuite.

On my workstation with the testsuite (of course):
```
2026-01-25T08:37:10 >>> sys-libs/glibc-2.43.9999: 1:26:30
2026-01-26T03:11:15 >>> sys-libs/glibc-2.43.9999: 1:28:43
2026-02-10T08:58:06 >>> sys-libs/glibc-2.43.9999: 1:27:00
2026-02-18T16:03:23 >>> sys-libs/glibc-2.43.9999: 2:30:43
2026-02-26T15:57:14 >>> sys-libs/glibc-2.43.9999: 2:24:52
2026-03-02T22:43:30 >>> sys-libs/glibc-2.43.9999: 2:25:55
```

I upgraded to new gawk (a beta) on 2026-02-18T08:36:34.

Link: https://inbox.sourceware.org/libc-alpha/lhuikb5ibey.fsf@oldenburg.str.redhat.com/
Signed-off-by: Sam James <sam@gentoo.org>

sys-libs/glibc/glibc-2.43-r2.ebuild

@@ -1372,6 +1372,9 @@ glibc_src_test() {
 		done
 	fi
 
+	# https://inbox.sourceware.org/libc-alpha/lhuikb5ibey.fsf@oldenburg.str.redhat.com/
+	local -x GAWK_GNU_MATCHERS=1
+
 	# sandbox does not understand unshare() and prevents
 	# writes to /proc/, which makes many tests fail
 

sys-libs/glibc/glibc-2.43.9999.ebuild

@@ -1371,6 +1371,9 @@ glibc_src_test() {
 		done
 	fi
 
+	# https://inbox.sourceware.org/libc-alpha/lhuikb5ibey.fsf@oldenburg.str.redhat.com/
+	local -x GAWK_GNU_MATCHERS=1
+
 	# sandbox does not understand unshare() and prevents
 	# writes to /proc/, which makes many tests fail
 

sys-libs/glibc/glibc-9999.ebuild

@@ -1371,6 +1371,9 @@ glibc_src_test() {
 		done
 	fi
 
+	# https://inbox.sourceware.org/libc-alpha/lhuikb5ibey.fsf@oldenburg.str.redhat.com/
+	local -x GAWK_GNU_MATCHERS=1
+
 	# sandbox does not understand unshare() and prevents
 	# writes to /proc/, which makes many tests fail