breakpoints: fix build on -fno-inline

Some configuration of GCC could default to -fno-inline, causing
the build to fail after commit fb43f1ff4e2f ("target: Rework 'set'
variable of break-/watchpoints").

Switch the new inline functions to 'static inline', as it's widely
used in the rest of the code.

Change-Id: I8bf31045a137bd34ed825f4b2a9338eb3c70046d
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Fixes: fb43f1ff4e2f ("target: Rework 'set' variable of break-/watchpoints")
Reviewed-on: https://review.openocd.org/c/openocd/+/6881
Tested-by: jenkins