Add fast prctl implementation for i686/armv7

This gets me down to ~5-6 instructions of overhead on x86 and armv7:

```julia
julia> println(Sys.ARCH); @pstats "(instructions,branch-instructions,branch-misses)" nothing
arm
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
┌ instructions             6.00e+00  100.0%
│ branch-instructions      1.00e+00  100.0%  # 16.7% of insns
└ branch-misses            0.00e+00  100.0%  #  0.0% of branch insns
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
```

```julia
julia> println(Sys.ARCH); @pstats "(instructions,branch-instructions,branch-misses)" nothing
i686
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
┌ instructions             5.00e+00  100.0%
│ branch-instructions      2.00e+00  100.0%  # 40.0% of insns
└ branch-misses            1.00e+00  100.0%  # 50.0% of branch insns
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
```

Author: topolarity

src/LinuxPerf.jl

@@ -360,10 +360,18 @@ const PR_TASK_PERF_EVENTS_ENABLE = Cint(32)
         res = Base.llvmcall("""%val = call i64 asm sideeffect "syscall", "={rax},{rax},{rdi},~{rcx},~{r11},~{memory}"(i64 %0, i64 %1)
                                ret i64 %val""", Int64, Tuple{Int64, Int64}, SYS_prctl, Int64(op))
         return (res >= 0) ? nothing : throw(Base.SystemError("prctl", -res, nothing))
+    elseif Sys.ARCH == :i686
+        res = Base.llvmcall("""%val = call i32 asm sideeffect "int \$\$0x80", "={eax},{eax},{ebx},~{memory}"(i32 %0, i32 %1)
+                               ret i32 %val""", Int32, Tuple{Int32, Int32}, SYS_prctl, Int32(op))
+        return (res >= 0) ? nothing : throw(Base.SystemError("prctl", -res, nothing))
     elseif Sys.ARCH == :aarch64
         res = Base.llvmcall("""%val = call i64 asm sideeffect "svc #0", "={x0},{x8},{x0},~{memory}"(i64 %0, i64 %1)
                                ret i64 %val""", Int64, Tuple{Int64, Int64}, SYS_prctl, Int64(op))
         return (res >= 0) ? nothing : throw(Base.SystemError("prctl", -res, nothing))
+    elseif Sys.ARCH == :arm
+        res = Base.llvmcall("""%val = call i32 asm sideeffect "swi 0", "={r0},{r7},{r0},~{memory}"(i32 %0, i32 %1)
+                               ret i32 %val""", Int32, Tuple{Int32, Int32}, SYS_prctl, Int32(op))
+        return (res >= 0) ? nothing : throw(Base.SystemError("prctl", -res, nothing))
     else
         # syscall is lower overhead than calling libc's prctl
         res = ccall(:syscall, Cint, (Clong, Clong...), SYS_prctl, op)