[Threading] Use raw gettid syscall for older glibc compatibility
[Threading] Use raw gettid syscall for older glibc compatibility

`gettid` wrapper was added in glibc 2.30, and Ubuntu 18.04 uses glibc
2.27, so use raw syscall instead of it.

benlangmuir/swift,benlangmuir/swift,glessard/swift,JGiola/swift,JGiola/swift,roambotics/swift,roambotics/swift,roambotics/swift,benlangmuir/swift,benlangmuir/swift,apple/swift,roambotics/swift,JGiola/swift,roambotics/swift,glessard/swift,JGiola/swift,glessard/swift,roambotics/swift,glessard/swift,benlangmuir/swift,JGiola/swift,apple/swift,JGiola/swift,benlangmuir/swift,apple/swift,apple/swift,glessard/swift,apple/swift,glessard/swift,apple/swift