rtos: Create a new helper function find_symbol(...)

This will be utilized for an upcoming refactorization to support -flto
compiled programs.

Signed-off-by: Tim Nordell <tnordell@airgain.com>
Change-Id: Id523c0b3ac2dad8b248ea0d2cac7b4dd2f83d293
Reviewed-on: https://review.openocd.org/c/openocd/+/7177
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>