Fix broken pipe caused by commands in toolchain.gypi.

I sometimes observe broken pipe errors when I run gyp, which causes the gyp
process to fail. This seems to happen if gyp is executed under heavy I/O
pressure.

This change removes the pipes by replacing "echo |" with "< /dev/null".
Technically these are not the same, because "echo |" prints a newline while
"< /dev/null" doesn't, but this difference is not significant in the context of
the commands in toolchain.gypi.

BUG=
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/73623002

Patch from Yuta Kitamura <yutak@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17836 ce2b1a6d-e550-0410-aec6-3dcde31c8c00