Refactor simplify `Process::Status#exit_reason` on Unix (#15288) #3546

Job	Run time
check_format	1m 35s
x86_64-gnu-test-preview_mt	5m 55s
x86_64-gnu-test (1.7.3)	33m 11s
x86_64-musl-test	34m 29s
x86_64-gnu-test (1.8.2)	35m 7s
x86_64-gnu-test (1.9.2)	33m 15s
x86_64-gnu-test (1.10.1)	36m 42s
x86_64-gnu-test (1.11.2)	33m 34s
x86_64-gnu-test (1.12.2)	35m 6s
x86_64-gnu-test (1.13.3)	32m 10s
x86_64-gnu-test (1.14.0)	33m 49s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	32m 23s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	32m 42s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	32m 21s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	34m 16s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	32m 24s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	31m 50s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	32m 0s
	9h 2m 49s

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