MIPS: compute num_got_entries from .got section's size

The '__got_start' and '__got_end' symbols are used
only in the linker script to compute the value of
the 'num_got_entries' symbol.

Remove the symbols and use the SIZEOF(.got) command
to get the size of the .got section.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>