media-libs/libpng: Disable RISC-V V extension

Testing the latest release on a machine with C920 core fails with a lot
of `Illegal instruction     (core dumped) "$@"` messages even with the
locally applied post-release fixes, and the upstream configure seems
to have harcoded the -march flag to turn the V extension on even when
it's off in the build system which won't work for systems that don't
support V extension, so disable it completely for now.

Closes: https://bugs.gentoo.org/958203
Signed-off-by: Jakov Smolić <jsmolic@gentoo.org>

media-libs/libpng/libpng-1.6.49.ebuild

@@ -65,7 +65,7 @@ multilib_src_configure() {
 		$(use_enable test tests)
 		$(use_enable cpu_flags_x86_sse intel-sse)
 		$(use_enable static-libs static)
-		--enable-riscv-rvv=check
+		--disable-riscv-rvv
 	)
 
 	ECONF_SOURCE="${S}" econf "${myeconfargs[@]}"