runners: jlink: Add support for big endian device

Sets the endianness of the JLink and the GDB Server based on if
`CONFIG_BIG_ENDIAN` is set.

Doc on JLink LE/BE option:
https://wiki.segger.com/J-Link_Commander#LE

Doc on GDB server -endian option:
https://wiki.segger.com/J-Link_GDB_Server#-endian



Signed-off-by: Sigmund Klåpbakken <sigmundklaa@outlook.com>