Moves the ets_timer_arm() / ets_timer_disarm() code paths to RAM
Overhead is 740 bytes of IRAM, 0 bytes DRAM
(For comparison: If all of esp_timer.c is moved to RAM, overhead is 1068 bytes IRAM and 480 bytes DRAM.)
Since timestamps are 64-bit, loosing one bit of range due to sign does
not present an issue, however for applications doing calculations on
timestamps, signed return type is more convenient.