add a switch for libm. #296

Job	Run time
build-native-ubuntu-24.04-gcc	3m 24s
build-native-ubuntu-24.04-llvm	3m 31s
build-native-ah-ubuntu_22_04-c7g_2x-50-gcc	2m 2s
build-native-ah-ubuntu_22_04-c7g_2x-50-llvm	2m 54s
test-native-ubuntu-24.04-gcc	8m 52s
build-armhf-gcc	2m 39s
test-native-ubuntu-24.04-llvm	8m 12s
build-armhf-llvm	1m 32s
test-native-ah-ubuntu_22_04-c7g_2x-50-gcc	2m 49s
build-ppc64el-gcc	2m 40s
test-native-ah-ubuntu_22_04-c7g_2x-50-llvm	2m 22s
build-ppc64el-llvm	2m 39s
build-s390x-gcc	2m 5s
build-s390x-llvm	2m 52s
build-riscv64-gcc	3m 49s
build-riscv64-llvm	2m 46s
test-armhf-gcc (qemu_cpu: "max")	12m 55s
test-armhf-llvm (qemu_cpu: "max")	13m 18s
test-ppc64el-gcc (qemu_cpu: "power10")	26m 17s
test-ppc64el-llvm (qemu_cpu: "power10")	24m 9s
test-s390x-gcc (qemu_cpu: "")	19m 57s
test-s390x-llvm (qemu_cpu: "")	18m 46s
test-riscv64-gcc (qemu_cpu: "rv64,zba=true,zbb=true,zbs=true,v=false")	13m 14s
test-riscv64-gcc (qemu_cpu: "rv64,zba=true,zbb=true,zbs=true,v=true,vlen=128,elen=64,vext_spec=v1.0")	36m 38s
test-riscv64-gcc (qemu_cpu: "rv64,zba=true,zbb=true,zbs=true,v=true,vlen=256,elen=64,vext_spec=v1.0")	50m 44s
test-riscv64-llvm (qemu_cpu: "rv64,zba=true,zbb=true,zbs=true,v=false")	13m 17s
test-riscv64-llvm (qemu_cpu: "rv64,zba=true,zbb=true,zbs=true,v=true,vlen=128,elen=64,vext_spec=v1.0")	30m 27s
test-riscv64-llvm (qemu_cpu: "rv64,zba=true,zbb=true,zbs=true,v=true,vlen=256,elen=64,vext_spec=v1.0")	41m 17s
	5h 56m 7s

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