MIPS: fix linking of standalone programs

Use the global MIPS specific u-boot.lds for linking standalone programs
instead of the outdated ones in examples/standalone/. Also pass --gc-sections
in LDFLAGS_STANDALONE to optimize the size of standalone programs.
Finally remove the deprecated config.mk files in arch/mips/cpu/mips[32,64]/.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>