MIPS: compressed: fix build with enabled UBSAN

Commit 1e35918a ("MIPS: Enable Undefined Behavior Sanitizer
UBSAN") added a possibility to build the entire kernel with UBSAN
instrumentation for MIPS, with the exception for VDSO.
However, self-extracting head wasn't been added to exceptions, so
this occurs:

mips-alpine-linux-musl-ld: arch/mips/boot/compressed/decompress.o:
in function `FSE_buildDTable_wksp':
decompress.c:(.text.FSE_buildDTable_wksp+0x278): undefined reference
to `__ubsan_handle_shift_out_of_bounds'
mips-alpine-linux-musl-ld: decompress.c:(.text.FSE_buildDTable_wksp+0x2a8):
undefined reference to `__ubsan_handle_shift_out_of_bounds'
mips-alpine-linux-musl-ld: decompress.c:(.text.FSE_buildDTable_wksp+0x2c4):
undefined reference to `__ubsan_handle_shift_out_of_bounds'
mips-alpine-linux-musl-ld: arch/mips/boot/compressed/decompress.o:
decompress.c:(.text.FSE_buildDTable_raw+0x9c): more undefined references
to `__ubsan_handle_shift_out_of_bounds' follow

Add UBSAN_SANITIZE := n to mips/boot/compressed/Makefile to exclude
it from instrumentation scope and fix this issue.

Fixes: 1e35918a ("MIPS: Enable Undefined Behavior Sanitizer UBSAN")
Cc: stable@vger.kernel.org # 5.0+
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>