fix checkptr in (*Mem).Get on darwin on Go 1.22

[stevendanna]

The unit tests currently fail under race on Go 1.22 on Darwin.

> go test -race -v -run TestMem

fatal error: checkptr: converted pointer straddles multiple allocations

goroutine 35 gp=0xc000104c40 m=0 mp=0x104af4d00 [running]:
runtime.throw({0x1048d66ee?, 0xc00012c7d8?})
        /opt/homebrew/Cellar/go/1.22.1/libexec/src/runtime/panic.go:1023 +0x40 fp=0xc000067cc0 sp=0xc000067c90 pc=0x1046b8a20
runtime.checkptrAlignment(0x1048ca639?, 0xa?, 0x104af4d00?)
        /opt/homebrew/Cellar/go/1.22.1/libexec/src/runtime/checkptr.go:26 +0x70 fp=0xc000067ce0 sp=0xc000067cc0 pc=0x1046861e0
github.com/elastic/gosigar.sysctlbyname({0x1048ca639, 0xa}, {0x10494bca0, 0xc000067e30})
        /Users/ssd/src/gosigar/sigar_common_darwin.go:483 +0xa8 fp=0xc000067d60 sp=0xc000067ce0 pc=0x1048800d8
github.com/elastic/gosigar.(*Mem).Get(0xc000067e30)
        /Users/ssd/src/gosigar/sigar_common_darwin.go:48 +0x78 fp=0xc000067df0 sp=0xc000067d60 pc=0x10487d2a8
github.com/elastic/gosigar_test.TestMem(0xc0001349c0)
        /Users/ssd/src/gosigar/sigar_interface_test.go:37 +0x3c fp=0xc000067ed0 sp=0xc000067df0 pc=0x10488242c
testing.tRunner(0xc0001349c0, 0x104996310)
        /opt/homebrew/Cellar/go/1.22.1/libexec/src/testing/testing.go:1689 +0x184 fp=0xc000067fa0 sp=0xc000067ed0 pc=0x10478d1c4
testing.(*T).Run.gowrap1()
        /opt/homebrew/Cellar/go/1.22.1/libexec/src/testing/testing.go:1742 +0x44 fp=0xc000067fd0 sp=0xc000067fa0 pc=0x10478e664
runtime.goexit({})
        /opt/homebrew/Cellar/go/1.22.1/libexec/src/runtime/asm_arm64.s:1222 +0x4 fp=0xc000067fd0 sp=0xc000067fd0 pc=0x1046f5894
created by testing.(*T).Run in goroutine 1
        /opt/homebrew/Cellar/go/1.22.1/libexec/src/testing/testing.go:1742 +0x5e8

This is because syscall.Sysctl assumes that all return values are C strings and truncates the final byte if it is NUL. For Sysctl's that return integers, attempting to interpret the 7 byte buffer results in the checkptr violation.

Here, we fix this by using the sys/unix packages SysctlUint64.

[andrewkroh]

Thanks!