A few improvements for riscv #15025

Job	Run time
aux	4m 36s
dashboard	2m 53s
build	13m 30s
arch (presubmit_arch_linux)	10m 55s
gvisor	4m 28s
arch (presubmit_arch_freebsd)	6m 33s
old	6m 39s
race	8m 44s
arch (presubmit_arch_netbsd)	4m 36s
arch (presubmit_arch_openbsd)	4m 22s
race_dashboard	4m 59s
arch (presubmit_arch_darwin)	4m 26s
arch (presubmit_arch_windows)	3m 26s
arch (presubmit_arch_executor)	3m 20s
	1h 23m 27s

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