csu: test: explicitly add libm as build parameter

CSU tests build fails with '/usr/lib/libgcc_s.so: undefined reference to
fma' when built with LLVM 14 for powerpcspe, so '-lm' is being added
explicitly.

It may be linked to https://reviews.llvm.org/D77558

Reviewed by:	imp (earlier version)
MFC after:	2 days
Sponsored by:	Instituto de Pesquisas Eldorado (eldorado.org.br)
Differential Revision:	https://reviews.freebsd.org/D35691

lib/csu/tests/dynamic/Makefile

@@ -2,6 +2,11 @@
 
 .PATH: ${.CURDIR:H}
 
+.if exists(./Makefile.${MACHINE_ARCH})
+.include "./Makefile.${MACHINE_ARCH}"
+.endif
+
+
 .include <src.opts.mk>
 MK_PIE=	no
 

lib/csu/tests/dynamic/Makefile.powerpcspe

@@ -0,0 +1,11 @@
+# $FreeBSD$
+
+# TODO: investigate and try a better fix on compiler side
+#
+# CSU tests build fails with '/usr/lib/libgcc_s.so: undefined reference to fma'
+# when built with LLVM 14, so '-lm' is being added explicitly here. Only
+# the powerpcspe target is affected and it may be linked to
+#       https://reviews.llvm.org/D77558
+
+LDFLAGS+=	-lm
+