Switch to libunwind to remove dependency on libgcc for musl

The libgcc dependency is only used for _Unwind* symbols, remove
it now that libc_musl.so embeds libunwind.a.

Bug: 215802826
Test: m USE_HOST_MUSL=true host-native
Change-Id: I1ad4c1435fc0090c6df2d50d542d2203dfb94d7b

cc/config/x86_linux_host.go

@@ -65,7 +65,6 @@ var (
 
 	linuxMuslLdflags = []string{
 		"-nostdlib",
-		"-lgcc", "-lgcc_eh",
 		"--sysroot /dev/null",
 	}
 

rust/config/x86_linux_host.go

@@ -42,8 +42,6 @@ var (
 		"-nodefaultlibs",
 		"-nostdlib",
 		"-Wl,--no-dynamic-linker",
-		// for unwind
-		"-lgcc", "-lgcc_eh",
 	}
 	linuxX86Rustflags   = []string{}
 	linuxX86Linkflags   = []string{}