external/boringssl: work around Clang's lack of adrl.

This change works around Clang's lack of support for the adrl
pseudo-instruction by disabling Clang's as for BoringSSL.

See https://android-review.googlesource.com/#/c/150503/ for an
alternative solution that was discarded.

Change-Id: I1587376f8d864b7ea0c1fc953c7ea8a8552146e6
1 file changed