syscall: make use of signed char explicit in generating z-files on freebsd/arm

This CL is in preparation to make cgo work on freebsd/arm.

The signedness of C char might be a problem when we make bare syscall
APIs, Go structures, using built-in bootstrap scripts with cgo because
they do translate C stuff to Go stuff internally. For now almost all
the C compilers assume that the type of char will be unsigned on arm
by default but it makes a different view from amd64, 386.

This CL just passes -fsigned-char, let the type of char be signed,
option which is supported on both gcc and clang to the underlying C
compilers through cgo for avoiding such inconsistency on syscall API.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/59740051

src/pkg/syscall/mkall.sh

@@ -148,7 +148,9 @@ freebsd_arm)
 	mkerrors="$mkerrors"
 	mksyscall="./mksyscall.pl -l32 -arm"
 	mksysnum="curl -s 'http://svn.freebsd.org/base/head/sys/kern/syscalls.master' | ./mksysnum_freebsd.pl"
-	mktypes="GOARCH=$GOARCH go tool cgo -godefs"
+	# Let the type of C char be singed for making the bare syscall
+	# API consistent across over platforms.
+	mktypes="GOARCH=$GOARCH go tool cgo -godefs -- -fsigned-char"
 	;;
 linux_386)
 	mkerrors="$mkerrors -m32"