[lldb] Remove pre GCC 4.8 workaround (#73981)

The minimum GCC version was bumped up from 4.8 to 5.1 and then even newer
awhile ago so garbage collect the pre 4.8 workaround.

https://reviews.llvm.org/D66188

Author: brad0

lldb/source/Host/common/Host.cpp

@@ -133,11 +133,7 @@ class ScopedPThreadCancelDisabler {
 #endif // __linux__
 
 #ifdef __linux__
-#if defined(__GNUC__) && (__GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 8))
-static __thread volatile sig_atomic_t g_usr1_called;
-#else
 static thread_local volatile sig_atomic_t g_usr1_called;
-#endif
 
 static void SigUsr1Handler(int) { g_usr1_called = 1; }
 #endif // __linux__