Auto merge of #41420 - alexcrichton:mingw-6.3.0, r=petrochenkov

Auto merge of #41420 - alexcrichton:mingw-6.3.0, r=petrochenkov

appveyor: Upgrade to gcc for mingw 6.3.0

This commit sort of brings back #40777 by upgrading back to 6.3.0. While
investigating #40546 it was discovered that 6.3.0 appears to not spurious
fail in the same way that 6.2.0 does (which we're currently using). The
workaround for #40184 contained in #40777 did not work so this commit also
contains a different workaround for the gdb issue. We will not download the
6.2.0 version of gdb and use that instead of the default version that comes with
6.3.0.

I'm going to optimistically say...

Closes #40546