Makefile.system: -lm is necessary on power as well #2334

Job	Run time
TEST (LA464, NO_SHARED=1 DYNAMIC_ARCH=1 TARGET=LA464)	15m 17s
TEST (LOONGSON3R5, NO_SHARED=1 DYNAMIC_ARCH=1 TARGET=LOONGSON3R5)	15m 9s
TEST (LA264, NO_SHARED=1 DYNAMIC_ARCH=1 TARGET=LA264)	14m 46s
TEST (LOONGSON2K1000, NO_SHARED=1 DYNAMIC_ARCH=1 TARGET=LOONGSON2K1000)	14m 20s
TEST (LOONGSONGENERIC, NO_SHARED=1 DYNAMIC_ARCH=1 TARGET=LOONGSONGENERIC)	13m 46s
TEST (DYNAMIC_ARCH, NO_SHARED=1 DYNAMIC_ARCH=1 TARGET=GENERIC)	15m 9s
TEST (LA64_GENERIC, NO_SHARED=1 DYNAMIC_ARCH=1 TARGET=LA64_GENERIC)	15m 2s
	1h 43m 29s

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