Allow overriding -fno-gnu-inline-assembly

While -fno-gnu-inline-assembly is default, in some cases we need want
override the option and explicitly enable inline assembly.

BUG=https://bugs.chromium.org/p/nativeclient/issues/detail?id=4347
R=dschuff@chromium.org

Review URL: https://codereview.chromium.org/1767393002 .