MIPS: boot/compressed/: add __ashldi3 to target for ZSTD compression

Just like before with __bswapdi2(), for MIPS pre-boot when
CONFIG_KERNEL_ZSTD=y the decompressor function will use __ashldi3(), so
the object file should be added to the target object file.

Fixes these build errors:

mipsel-linux-ld: arch/mips/boot/compressed/decompress.o: in function `FSE_buildDTable_internal':
decompress.c:(.text.FSE_buildDTable_internal+0x48): undefined reference to `__ashldi3'
mipsel-linux-ld: arch/mips/boot/compressed/decompress.o: in function `FSE_decompress_wksp_body_default':
decompress.c:(.text.FSE_decompress_wksp_body_default+0xa8): undefined reference to `__ashldi3'
mipsel-linux-ld: arch/mips/boot/compressed/decompress.o: in function `ZSTD_getFrameHeader_advanced':
decompress.c:(.text.ZSTD_getFrameHeader_advanced+0x134): undefined reference to `__ashldi3'

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>