Include AIX in FromStr for Signal impl for SIGPWR #2104

Job	Run time
macos-aarch64	3m 51s
macos	6m 17s
aarch64_linux_native_builds (aarch64-unknown-linux-gnu)	4m 21s
minver	17s
x86_64_linux_native_builds (x86_64-unknown-linux-musl)	4m 15s
tier3 (x86_64-unknown-openbsd)	42s
solaris (x86_64-pc-solaris)	4m 23s
tier3 (x86_64-unknown-haiku)	40s
aarch64_linux_native_builds (aarch64-unknown-linux-musl)	4m 42s
x86_64_linux_native_builds (x86_64-unknown-linux-gnu)	4m 28s
rust_stable	1m 33s
tier3 (armv7-unknown-linux-uclibceabihf)	47s
tier3 (x86_64-unknown-dragonfly)	44s
rustfmt	14s
cross (i686-unknown-linux-gnu)	2m 1s
cross (powerpc64le-unknown-linux-gnu)	1m 56s
cross (armv7-unknown-linux-gnueabihf)	1m 58s
cross (i686-unknown-linux-musl)	1m 58s
cross_compiling (aarch64-linux-android)	36s
cross (loongarch64-unknown-linux-gnu)	1m 57s
redox	41s
cross (arm-unknown-linux-gnueabi)	1m 58s
cross_compiling (armv7-linux-androideabi)	39s
cross_compiling (arm-linux-androideabi)	39s
cross_compiling (armv7-unknown-linux-ohos)	40s
cross_compiling (x86_64-linux-android)	40s
cross_compiling (i686-linux-android)	38s
cross_compiling (aarch64-unknown-linux-ohos)	39s
cross_compiling (powerpc64-unknown-linux-gnu)	45s
cross_compiling (aarch64-apple-ios, true)	31s
cross_compiling (x86_64-unknown-netbsd)	35s
cross_compiling (x86_64-unknown-fuchsia)	33s
cross_compiling (x86_64-unknown-linux-ohos)	41s
cross_compiling (arm-unknown-linux-musleabi)	43s
cross_compiling (x86_64-unknown-linux-gnux32)	43s
cross_compiling (x86_64-unknown-illumos)	51s
cross_compiling (s390x-unknown-linux-gnu)	44s
	1h 0m 20s

Provide feedback