Set aarch64-fix-cortex-a53-835769 as default for AArch64

See previous CLs for details about Cortex-A53 erratum (835769)
Flag aarch64-fix-cortex-a53-835769 is set to 1 or 0 by clang,
depending on the presence of -mfix-cortex-a53-835769 or
-mno-fix-cortex-a53-835769.  If neither is given,
this CL changes the default to "true" for aarch64-fix-cortex-a53-835769.

It makes sense to have this workaround enabled by default in Android
when apps generated by NDK toolchains can potentially run on affected A53.
Android platform clang can specify -mno-fix-cortex-a53-835769 to disable this
workaround for A53 known to be not affected.

This changes also match the behavior in GCC r216079 when it's configured with
--enable-fix-cortex-a53-835769, which is the case for NDK toolchains.

Change-Id: I56d6c273f79c25d18e2cdd1b61015f35082bbb23