Fix: platform impl does not require floating point

Build RustSBI using RISC-V RV64IMAC instruction set

platform/k210/.cargo/config.toml

@@ -1,7 +1,7 @@
 [build]
-target = "riscv64gc-unknown-none-elf"
+target = "riscv64imac-unknown-none-elf"
 
-[target.riscv64gc-unknown-none-elf]
+[target.riscv64imac-unknown-none-elf]
 rustflags = [
     "-C", "link-arg=-Tlink-k210.ld",
 ]

platform/qemu/.cargo/config.toml

@@ -1,7 +1,7 @@
 [build]
-target = "riscv64gc-unknown-none-elf"
+target = "riscv64imac-unknown-none-elf"
 
-[target.riscv64gc-unknown-none-elf]
+[target.riscv64imac-unknown-none-elf]
 rustflags = [
     "-C", "link-arg=-Tlink-qemu.ld",
 ]