Build sdk variants of libc++ for riscv64

There is no prebuilt libc++ for riscv64 provided by the NDK, build
it using the platform sources instead.

Test: lunch aosp_riscv64-userdebug && m -k
bbg

Change-Id: I3e443925549d04edebc9adc56116c5254dbf4307
1 file changed