fix link issues on aarch64 musl

The linker was unable to find __addtf3, __multf3 and __subtf3.

Added target-feature=+crt-static and link-arg=-lgcc as a temporary
workaround. This seems to be the accepted fix in the Rust community:
rust-lang/compiler-builtins#201

A permanent fix is yet to be implemented in the Rust compiler.

Signed-off-by: Andreea Florescu <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

Author: andreeaflorescu

.cargo/config

@@ -0,0 +1,2 @@
+[target.aarch64-unknown-linux-musl]
+rustflags = [ "-C", "target-feature=+crt-static", "-C", "link-arg=-lgcc"]