As discussed and agreed in Linaro Connect Budapest 2017 [1],
the GNU name for ARM64 with 32-bit ABI has been changed, e.g.,
from:
aarch64_ilp32-linux-gnu
to:
aarch64-linux-gnu_ilp32
The current code has "ilp32" as an OS variant; this change
updates that variant to include the underscore in order to
match the agreed convention.
More information about ARM64 ILP32 can be found here:
* https://wiki.debian.org/Arm64ilp32Port
* https://wiki.linaro.org/Platform/arm64-ilp32
