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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