[libc] Use __builtin_ffsll for RPC lane mask (#85000)

src/__support/GPU/utils.h doesn't compile on a 32-bit platforms because
__builtin_ffsl uses long which is a 32-bit number. Use __builtin_ffsll
which uses long long which is guaranteed to be at least 64-bits.

GitOrigin-RevId: c6a93fe80b3cf30ff82d06e959c1177798c858ae
1 file changed