Pick armv7 using '-march=armv7-a'

Bug: http://b/68816454

Instead of the armv7-linux triple, use -march=armv7-a so we don't have
to special-case for arm when installing runtimes into the resource dir.
The driver already normalizes armv7 and variants to 'arm' while looking
for an arch-specific subdir in the resource dir.

Test: Build toolchain and verify installation path
Change-Id: I6927c31dc4e177f02721b5f9e9136780ce1db974