libcxxrt: allow build with gcc13 and --no-undefined-version

GCC 13 supports _Float16 and __int128 on fewer architectures than clang
and thus libcxxrt compiled with gcc13 is sometimes missing related
symbols.  Hack around this by explicitly appending --undefined-version
to LDFLAGS in problematic cases.

Reviewed by:	theraven, dim
Differential Revision:	https://reviews.freebsd.org/D45233

lib/libcxxrt/Makefile

@@ -42,3 +42,13 @@ Version-64.map: Version.map
 .endif
 
 .include <bsd.lib.mk>
+
+# gcc13 doesn't support _Float16 on riscv, powerpc64, and x86 or __int128
+# on i386.  Disable checks for missing symbols to allow this to link.
+#
+# XXX: This is a hack that hides an ABI divergence between clang and gcc.
+.if ${COMPILER_TYPE} == "gcc" && \
+    (${MACHINE_ARCH} == "amd64" || ${MACHINE_ARCH} == "i386" || \
+     ${MACHINE_ARCH:Mpowerpc64*} || ${MACHINE_ARCH} == "riscv64")
+LDFLAGS+=	-Wl,--undefined-version
+.endif