POSIX also says that ucontext_t's uc_sigmask has type sigset_t.
MIPS64 strace needs this.
The #define is to keep chromium off our lawn; otherwise it tries to redefine
all this stuff itself. We should probably clean that up and remove the #define.
(cherry picked from commit 26a8eb50a84e131d34d10d5d167d67e9995399bd)
Bug: 18172268
Change-Id: I49d7d09dabfc6c6926a8e1f4b235d041e2f2fc4d