tools/build: Don't redefine open() for the linux bootstrap

This is needed to bootstrap llvm-tblgen on Linux since LLVM calls
::open(...) which does not work if open is a statement macro.
Also stop defining O_SHLOCK/O_EXLOCK and update the only bootstrap tools
user of those flags to deal with missing definitions.

Reviewed By: jrtc27
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31226

(cherry picked from commit 5f6c8ce2452da2da233e37bf4c2b6fccde8594b1)