Makefile.power: allow using power10 instructions #2640

Job	Run time
TEST (RISCV64_ZVL256B, TARGET=RISCV64_ZVL256B BINARY=64 ARCH=riscv64 BUILD_BFLOAT16=1 BUILD_HFLOA...	15m 32s
TEST (RISCV64_ZVL128B, TARGET=RISCV64_ZVL128B BINARY=64 ARCH=riscv64, rv64,g=true,c=true,v=true,v...	14m 37s
TEST (DYNAMIC_ARCH=1, TARGET=RISCV64_GENERIC BINARY=64 ARCH=riscv64 DYNAMIC_ARCH=1, rv64,g=true,c...	7m 32s
	37m 41s

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