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The header is included for __NR_SYSCALL_BASE and __ARM_NR_BASE.
Some toolchain (in particular, PNaCl toolchain) does not provide
asm/unistd.h (so that neither __NR_SYSCALL_BASE nor __ARM_NR_BASE
is provided).
This CL removes the dependency to asm/unistd.h from linux_syscall_ranges.h,
instead define the values if necessary.

Also, this CL removes the include from syscall.cc, too, which is
unused.

TEST=Ran bots.
BUG=358465

Review URL: https://codereview.chromium.org/1071653002

Cr-Commit-Position: refs/heads/master@{#324371}