[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.

Author: boomanaiden154

llvm/tools/llvm-exegesis/lib/SubprocessMemory.cpp

@@ -24,6 +24,15 @@ namespace exegesis {
 
 #if defined(__linux__)
 
+// The SYS_* macros for system calls are provided by the libc whereas the
+// __NR_* macros are from the linux headers. This means that sometimes
+// SYS_* macros might not be available for certain system calls depending
+// upon the libc. This happens with the gettid syscall and bionic for
+// example, so we use __NR_gettid when no SYS_gettid is available.
+#ifndef SYS_gettid
+#define SYS_gettid __NR_gettid
+#endif
+
 long SubprocessMemory::getCurrentTID() {
   // We're using the raw syscall here rather than the gettid() function provided
   // by most libcs for compatibility as gettid() was only added to glibc in