posix, managarm: implement sigpending #3237

Job	Run time
Compile sysdeps (dripos)	58s
Build mlibc (x86_64, mlibc)	9m 12s
Compile sysdeps (lemon)	59s
Build mlibc (x86_64, mlibc-static)	1m 55s
Compile sysdeps (aero)	1m 4s
Build mlibc (x86_64, mlibc-shared)	1m 59s
Compile sysdeps (ironclad)	1m 4s
Build mlibc (x86_64, mlibc-ansi-only)	1m 48s
Compile sysdeps (lyre)	1m 1s
Build mlibc (x86_64, mlibc-headers-only)	1m 3s
Compile sysdeps (keyronex)	1m 33s
Build mlibc (riscv64, mlibc)	2m 22s
Build mlibc (riscv64, mlibc-static)	3m 17s
Build mlibc (riscv64, mlibc-shared)	1m 58s
Build mlibc (riscv64, mlibc-ansi-only)	1m 46s
Build mlibc (riscv64, mlibc-headers-only)	1m 10s
Build mlibc (aarch64, mlibc)	2m 51s
Build mlibc (aarch64, mlibc-static)	2m 9s
Build mlibc (aarch64, mlibc-shared)	2m 6s
Build mlibc (aarch64, mlibc-ansi-only)	2m 3s
Build mlibc (aarch64, mlibc-headers-only)	1m 11s
	43m 29s

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