boringssl_self_test: Enable optimization.

libcrypto performs a self test when it is loaded, unless
a marker file /dev/boringssl/selftest/[hash] exists which
indicates that the self test has already successfully
completed since the last time the device was booted.

Before this CL topic, libcrypto attempted to create the
marker file when the self test successfully completed.
On Android, dedicated boringssl_self_test{32,64} binaries
are run early during boot and are the only binaries
(apart from init and vendor_int) that have permission to
create these files.

Another CL in this topic stops the boringssl self test
creating a marker file unless the environment variable
BORINGSSL_SELF_TEST_CREATE_FLAG is set to a nonempty value.
This CL sets that value to "true" when running the dedicated
self test binaries, but not for other binaries. This has
the effect that other binaries that run the self test
early during boot (before the dedicated self test binaries
have created the marker files) and which run the self test
will no longer attempt to create the marker file, which
SELinux would have denied anyway.

Bug: 137267623
Test: Treehugger

Change-Id: I99317df1a8c3496d33ae83f9ec346782b2286ac9