remove the trailing back slash in Makefile.in #274

Job	Run time
test-sim (ubuntu-20.04, newlib, rv64gc-lp64d, spike)	2h 11m 13s
build (ubuntu-20.04, newlib, rv32gc-ilp32d, gcc)	3h 2m 10s
build (ubuntu-20.04, newlib, rv32gc-ilp32d, llvm)	3h 33m 35s
build (ubuntu-20.04, newlib, rv64gc-lp64d, gcc)	2h 57m 17s
build (ubuntu-20.04, newlib, rv64gc-lp64d, llvm)	3h 6m 44s
build (ubuntu-20.04, linux, rv32gc-ilp32d, gcc)	5h 1m 2s
build (ubuntu-20.04, linux, rv32gc-ilp32d, llvm)	4h 37m 55s
build (ubuntu-20.04, linux, rv64gc-lp64d, gcc)	3h 46m 37s
build (ubuntu-20.04, linux, rv64gc-lp64d, llvm)	3h 40m 23s
build (ubuntu-20.04, musl, rv64gc-lp64d, gcc)	51m 27s
build (ubuntu-22.04, newlib, rv32gc-ilp32d, gcc)	54m 51s
build (ubuntu-22.04, newlib, rv32gc-ilp32d, llvm)	3h 2m 9s
build (ubuntu-22.04, newlib, rv64gc-lp64d, gcc)	1h 13m 15s
build (ubuntu-22.04, newlib, rv64gc-lp64d, llvm)	2h 47m 16s
build (ubuntu-22.04, linux, rv32gc-ilp32d, gcc)	1h 8m 59s
build (ubuntu-22.04, linux, rv32gc-ilp32d, llvm)	4h 7m 20s
build (ubuntu-22.04, linux, rv64gc-lp64d, gcc)	1h 2m 54s
build (ubuntu-22.04, linux, rv64gc-lp64d, llvm)	3h 11m 38s
build (ubuntu-22.04, musl, rv64gc-lp64d, gcc)	45m 46s
build-multilib	0s
	2d 3h 2m 31s

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