Optimize ZROT_RVV for the unit-stride case (inc_x = inc_y = 1) #6287

Job	Run time
cross_build (riscv64, riscv64-linux-gnu, TARGET=RISCV64_GENERIC)	7m 46s
cross_build (alpha, alpha-linux-gnu, TARGET=EV4)	7m 46s
build (ubuntu-latest, gfortran, make)	21m 9s
build (ubuntu-latest, gfortran, cmake)	46m 0s
cross_build (mipsel, mipsel-linux-gnu, TARGET=MIPS1004K)	8m 54s
cross_build (mips64el, mips64el-linux-gnuabi64, DYNAMIC_ARCH=1 TARGET=GENERIC)	9m 13s
build (ubuntu-latest, flang, make)	17m 42s
build (macos-latest, gfortran, make)	14m 43s
msys2 (UCRT64, int32, Release)	1h 48m 9s
msys2 (CLANG64, int32, Release)	1h 8m 9s
msys2 (UCRT64, int64, Release)	1h 45m 57s
build (macos-latest, gfortran, cmake)	35m 4s
msys2 (CLANG64, int64, Release)	1h 17m 28s
msys2 (MINGW32, int32, Release)	1h 14m 53s
build (ubuntu-latest, flang, cmake)	44m 16s
neoverse_build	8m 18s
msys2 (UCRT64, int32, None, mingw-w64-ucrt-x86_64, fc)	1h 36m 14s
	12h 31m 41s

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