linux: reduce binary size in the common case

This commit changes the runtime.putchar implementation to directly call
the `write` system call. This reduces the binary size by around 2.7kB.

src/runtime/runtime_unix.go

@@ -7,8 +7,8 @@ import (
 	"unsafe"
 )
 
-//export putchar
+//export write
-func _putchar(c int) int
+func libc_write(fd int32, buf unsafe.Pointer, count uint) int
 
 //export usleep
 func usleep(usec uint) int
@@ -145,7 +145,8 @@ func syscall_runtime_envs() []string {
 }
 
 func putchar(c byte) {
-	_putchar(int(c))
+	buf := [1]byte{c}
+	libc_write(1, unsafe.Pointer(&buf[0]), 1)
 }
 
 func ticksToNanoseconds(ticks timeUnit) int64 {