[llvm-exegesis] Define SYS_gettid if not available

This patch defines SYS_gettid as __NR_gettid if SYS_gettid is not
available to avoid compile time errors due to SYS_gettid not being
defined. This happens with certain libcs (like bionic) that do not
define SYS_gettid.

GitOrigin-RevId: 61f400165c64894374d23ffbcbebc8650d974961
1 file changed