abis/linux: define sigcontext for x86(_64) #4278

Job	Run time
Compile sysdeps (dripos)	2m 18s
Build mlibc (x86_64, mlibc, gcc)	2m 18s
Compile sysdeps (lemon)	2m 21s
Build mlibc (x86_64, mlibc-static, gcc)	2m 7s
Compile sysdeps (aero)	2m 19s
Compile sysdeps (ironclad)	2m 24s
Build mlibc (x86_64, mlibc-shared, gcc)	1m 43s
Compile sysdeps (lyre)	2m 29s
Compile sysdeps (keyronex)	2m 22s
Build mlibc (x86_64, mlibc-ansi-only, gcc)	2m 5s
Compile sysdeps (managarm)	2m 48s
Build mlibc (x86_64, mlibc-headers-only, gcc)	2m 3s
Compile sysdeps (vinix)	2m 19s
Build mlibc (riscv64, mlibc, gcc)	3m 8s
Compile sysdeps (astral)	2m 20s
Build mlibc (riscv64, mlibc-static, gcc)	2m 37s
Build mlibc (riscv64, mlibc-shared, gcc)	2m 51s
Build mlibc (riscv64, mlibc-ansi-only, gcc)	2m 57s
Build mlibc (riscv64, mlibc-headers-only, gcc)	2m 21s
Build mlibc (aarch64, mlibc, gcc)	4m 11s
Build mlibc (aarch64, mlibc-static, gcc)	3m 17s
Build mlibc (aarch64, mlibc-shared, gcc)	3m 26s
Build mlibc (aarch64, mlibc-ansi-only, gcc)	3m 10s
Build mlibc (aarch64, mlibc-headers-only, gcc)	2m 5s
Build mlibc (x86, mlibc, gcc)	1m 41s
Build mlibc (x86, mlibc-static, gcc)	49s
Build mlibc (x86, mlibc-shared, gcc)	23s
Build mlibc (x86, mlibc-ansi-only, gcc)	1m 14s
Build mlibc (x86, mlibc-headers-only, gcc)	49s
Build mlibc (m68k, mlibc, gcc)	3m 33s
Build mlibc (m68k, mlibc-static, gcc)	2m 59s
Build mlibc (m68k, mlibc-shared, gcc)	3m 4s
Build mlibc (m68k, mlibc-ansi-only, gcc)	2m 45s
Build mlibc (m68k, mlibc-headers-only, gcc)	1m 53s
Build mlibc (x86_64, mlibc, clang)	2m 39s
	1h 23m 48s

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