configure: MinGW respect --bindir argument

configure: MinGW respect --bindir argument

There are two cases that need to be accounted for when compiling QEMU
for MinGW32:
 1) A standalone distribution, where QEMU is self contained and
    extracted by the user, such as a user would download from the QEMU
    website. In this case, all the QEMU executable files should be
    rooted in $prefix to ensure they can be easily found by the user
 2) QEMU integrated into a distribution image/sysroot/SDK and
    distributed with other programs. In this case, the provided
    arguments for bindir/datadir/etc. should be respected as they for a
    Linux build.

Restructures the MinGW path configuration so that all of the paths
except bindir use the same rules as when building for other platforms.
This satisfies #2 and #1 since these files do not need to be directly in
$prefix anyway.

The handling for --bindir is changed so that it defaults to $prefix on
MinGW (maintaining the compatibility with #1), but if the user specifies
a specific path when configuring it can also satisfy #2.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Message-Id: <20210112210239.28836-1-JPEWhacker@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>