[gn build] Set llvm_have_mallinfo2 to true by default

If you are using an old enough glibc which doesn't have mallinfo2, set
`llvm_have_mallinfo2 = false`.

At this point it's likely that most people using the gn build are
compiling against a recent enough glibc (glibc 2.33 which was released
in Feb 2021).

Reviewed By: peterwaller-arm

Differential Revision: https://reviews.llvm.org/D126415