tsan: print signal num in errno spoiling reports

For errno spoiling reports we only print the stack
where the signal handler is invoked. And the top
frame is the signal handler function, which is supposed
to give the info for debugging.
But in same cases the top frame can be some common thunk,
which does not give much info. E.g. for Go/cgo it's always
runtime.cgoSigtramp.

Print the signal number.
This is what we can easily gather and it may give at least
some hints regarding the issue.

Reviewed By: melver, vitalybuka

Differential Revision: https://reviews.llvm.org/D121979