Call remote platform GetSupportedArchitectureAtIndex if connected to remote.

Updated PlatformLinux::GetSupportedArchitectureAtIndex to call the
PlatformRemoteGdbServer::GetSupportedArchitectureAtIndex if connected remotely.
This should return the correct thing for android (to fix those failing tests),
and is also working for mac to linux.

Test Plan:
./dotest.py $DOTEST_OPTS -t -p TestCallStdStringFunction.py
The above still passes when running mac->linux indicating it successfully
identified PlatformLinux as the target platform and converted the mmap options
correctly.

Differential Revision: http://reviews.llvm.org/D9672

llvm-svn: 237128